Metallography

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TEXT-BOOKS OF PHYSICAL
CHEMISTRY

Edited by Sir WILLIAM RAMSAY. K.C.B., F.R.S.

METALLOGRAPHY

Text-Books of Physical Chemistry.

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STOICHIOMETRY. By Sydnky Young, D.Sc, F.R.S., Pro-
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LONGMANS, GREEN, AND CO.

39, PATERNOSTER ROW, LONDON
NEW YORK, BOMBAY, AND CALCUTTA

METALLOGRAPHY

CECIL H. DESCH
D.Sc. (LoND.), Ph.D. (Wurzb.)

GRAHAM VOUNCf LECTURER IN METALLURGICAL CHEMISTRY
IN THE UNIVERSITY OP GLASGOW

WITH 14 FLA TES
AND loS DIAGRAMS IN IIJE TEXT

SECOND EDITION

LONGMANS, GREEN, AND CO.

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1913

All rights reserved.

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DEDICATED

TO

MY FATHER AND MOTHER

PREFACE

The study of metallic alloys by physical and miscroscopical
methods has reached so great a development in recent years
as to form a distinct branch of physical chemistry. In the
following pages I have sought to present an account of the
methods employed in this branch of study, and of the con-
clusions which have been reached, and also to indicate the
directions in which further research is needed. The attempt
has been made to discriminate, in the literature of the subject,
between investigations performed with the requisite care and
thoroughness, and those which, from the use of impure
materials in preparing the alloys, the examination of in-
sufficiently large quantities, or other causes, fail to reach the
standard of accuracy required in physico-chemical work.

The abbreviations employed in the footnotes are, in most
cases, those adopted by the Chemical Society, and the re-
mainder will, it is hoped, be self-explanatory. References to
Russian periodicals are only given if the investigation has not
been published in full in another language.

The whole of the photo-micrographs have been prepared in
the Metallurgical Laboratory of the University of Glasgow.

I take this opportunity of expressing my thanks to
Dr. J. G. Gray and Mr. A. D. Ross, of the Physical Laboratory
of this University, who have kindly read in proof the section on
the magnetic properties of alloys, to Prof A. K. Huntington
for facilities afforded me when working in the Metallurgical

VUl PREFACE

Laboratory of King's College, London, and to Messrs. R.
and J. Beck, Messrs. W. Watson & Sons, Messrs. Carl Zeiss,
London, Messrs. J. Carling & Sons, Middlesbrough, and
Messrs. \\\ W. Scott & Co., Glasgow, for the use of blocks
illustrating apparatus.

Lastly, I wish to acknowledge the constant assistance of
my wife, both in the experimental work and in the preparation
of the text.

C. H. D.

The University,
Glasgow.

NOTE TO THE SECOND EDITION

^^'HILST the general plan and arrangement of the first edition
of this work are unchanged, the text has been revised
throughout, for the purpose of removing errors, of incor-
porating the most important results of recent investigations,
and of completing the references to publications. The most
important changes have been made in the chapters dealing
with the physical properties of alloys (Chap. XIL) and with
the metallography of iron and steel (Chap. XV IL) The
appendix has been completely revised, and incorporates pub-
lications received down to the time of going to press.

C. H. D.

Glasgow,

March, 1913.

CONTENTS

CHAPTER I
Introduction

CHAPTER II

The Diagram of Thermal Equilibrium i2

CHAPTER III
The Diagram of Thermal Equilibrium (cotUhiueii) ... 43
Solid Solutions or Mixed Crystals.

CHAPTER IV

The Diagram of Thermal Equilibrium {conHnued) ... 65
Ternary and more Complex Systems.

CHAPTER V

The Diagram op Thermal Equilibrium {continued) ... 81
Metals which are only Partially Miscible in the
Liquid State.

CHAPTER VI
Practical Pyrometry and Thermal Analysis 94

CHAPTER VII
The Preparation of Micro-Sections .... ... 134

CHAPTER VIII ■
The Microscopical Examination of Prepared Sections . 154

ix

X CONTENTS

PAGK

CHAPTER IX
The Crysiat.i.ization of Metals and Alloys ... .175

CHAPTER X

Undercooling and the Metastable State 197

CHAPTER XI
Diffusion in the Solid State 216

CHAPTER XII

The Physical PRorERTiES of Alloys 230

Density — Thermal Expansibility — Hardness — Electri-
cal Conductivity — Thermo-Electric Power — Magnetic
Properties.

CHAPTER XIII
Electromotive Force and Corrosion 276

CHAPTER XIV
The Construction of the Equilibrium Diagram .... 299

CHAPTER XV

The Molecular Condition of Metals in Alloys and the

Nature of Inter-Metallic Compounds 328

CHAPTER XVI
The Plastic Deformation of Metals and Alloys ... 341

CHAPTER XVII
The Metallography of Iron and Steel 361

CHAPTER XVIII
The Metallography of Industrial Alloys 386

APPENDIX
List of Systems 399

INDEX

419

LIST OF PLATES

TO FACE PACK

c A. Surface of Tin Ingot 177

( B. Etched Surface of Tin .177

\ A. Alloy of Copper and Nickel 182

\ B. Brass 182

( A. Alloy of Copper and Silver 183

I B. Alloy of Copper and Antimony 183

- I A. EuTECTic Alloy of Copper and Phosphorus . 185

I B. EuTECTic Alloy of Copper and Phosphorus . 185

I A. Alloy of Bismuth and Tin 186

( B. Alloy of Bismuth and Lead 186

( A. Alloy of Copper and Antimony 187

( B. Alloy of Copper and Antimony ... .187

( A. Alloy of Copper and Antimony 188

t B. Alloy or Antimony and Tin 188

^ A. Alloy of Copper and Aluminium .... 190

^ B. Aluminium Bronze 190

c A. Alloy of Copper and Zinc 191

( B. Alloy of Lead, Tin, and Bismuth .... 191

I A. Diffusion of Zinc in Copper 221

i B. Diffusion of Zinc in Copper 221

^ A. Slip-bands in Lead 346

\ B. Pearlite 346

( A. Scratched Surface of Bismuth 355

t B. Partly Polished Surface of Bismuiii . . . 355

1 A. Soft Iron 369

I B. Soft Steel 369

( A. White Pig Iron 376

I B. Eutectic of White Pig Iron 376

xi

].

II.

ill.

V.
VL
VII.

vjir.

IX.
X.

XI.

XII.

XIII.

XIV.

METALLOGRAPHY

CHAPTER I

INTRODUCTION

Metallography may be defined as the study of the internal
structure of metals and alloys, and of its relation to their com-
position, and to their physical and mechanical properties. It
is a branch of physical chemistry, since the internal structure
depends on the physical and chemical conditions under which
the solid metal or alloy is formed, and the study of structure
presents itself as a department of the study of equilibrium in
heterogeneous systems. Whilst, however, physical chemistry
concerns itself in general only with the nature and relative
quantity of the phases in a system, and with the transformations
of energy which accompany chemical changes, metallography
takes into account a further variable, namely, the mechanical
arrangement of the component particles. It is thus intimately
connected with crystallography.

The consideration of metals and alloys as a class apart from
other mixtures and solutions which obey the same physico-
chemical laws is partly an historical accident, and arises partly
from the great importance of the metals in technical practice.
The needs of practical metallurgy, especially in the iron and
steel industries, have been the motive of the earliest, and of
many of the most important metallographic investigations. The
study of structure has proved itself an indispensable auxiliary
to chemical analysis in the scientific control of the metallurgical
industries, an auxiliary of which the applications become more
extensive and more important every year. But from the
standpoint of pure science, the identity of the relations in
metallic and non-metallic systems must not be overlooked.
T.P.C. I B

2 METALLOGRAPHY

Geologists and mineralogists are now making use of the
methods and results of metallography to study the formation
and metamorphosis of igneous rocks, whilst light is being
thrown from the same source into the hitherto obscure region
of the cements and slags, and the science is capable of still
further extension.

The word " metallography " was formerly used ^ to signify
the description of metals and their properties. In this sense it
is obsolete, although an isolated example of its use is found as
late as 1871." Its reintroduction to designate the microscopic
structure of metals and alloys dates only from 1892,' since
when it has been generally accepted, gradually receiving an
extension of meaning to include investigations by other than
microscopical means.

The examination of metals by means of the microscope, so
recent as a method of systematic research, was nevertheless
practised by several of the older investigators. As far back as
1665, Robert Hooke, in his Micrographia, described the appear-
ance of lead crystallizing from its alloy with silver, and further
described and drew the magnified surface of a polished steel
blade, adding some thoughtful remarks on the nature of
polish.*

Reaumur, in 1722," employed the microscope to examine
the fractured surfaces of steel and of white and grey cast-iron,
founding on the results which he obtained a method of dis-
tinguishing between irons subjected to different thermal treat-
ments. An extract will show the nature of his observations : —

P. 392 : "Si on examine les unes et les autres fontes au
microscope, les fontes bien blanches y parditront toujours d'une

' The earliest instance of its use given in the New English Dictionary,
Oxford, is dated 1721.

' T. A. Blyth, Metallography as a Separate Science, London, 1871.

• F. Osmond, Rapport prisenti d la commission des mithodes d'essais des
matiriaux, February, 1892.

* Robert Hooke, Micrographia ; or. Some Physiological Descriptions of
Minute Bodies made with Magnifying Glasses, with Observations and
Enquiries thereon, London, 1665.

' R. A. F. de Reaumur, VArt de convertir leferforgien acier, et I'art
d'adoucir le/er/ondu, Paris, 1722.

INTRODUCTION 3

tissure compacte, on y pourra observer quelques lames plattes
parsemdes, mais beaucoup plus petites que celles de I'acier, la
mSme loupe qui fait apercevoir celles dont sont composes les
grains d'un acier trempd peu chaud, ne feroit pas appercevoir
celles-cy. Les fontes grises paroissent au microscope d'un tissu
tenement spongieux, que tout semble un amas d'especes de
crystalisations, ou si Ton veut de brbssailles, des especes de
vegetations chimiques, faites d'une infinite de branchages entre-
lassds, mais composes chacun de petites lames agencies les
unes sur les autres."

Reaumur suggests in the same work, one of the most im-
portant in the early history of iron, a polyhedral arrangement
of the crystals, and puts forward a theory to explain the harden-
ing effect of quenching steel. The very numerous drawings
testify to the careful character of his observations.

The microscopical examination of fractured surfaces is of
very limited application, and is unsuitable for systematic study.
The way towards d better method was opened by the discovery
of Widmanstatten, in 1808,^ that certain meteorites when cut
and polished develop a distinct and characteristic structure on
being etched with acids, or oxidized by heating in air. Wid-
manstatten's figures being visible without magnification, the
process was not extended to metals having a more minute
structure, and metallography made no further progress for
many years. In 1864, H. C. Sorby, of Sheffield, who may
also be regarded as the founder of the modern science of
microscopical petrography, was led from the study of rocks
and meteorites to that of iron and steel. His early publica-
tions on the subject were confined to brief notes,^ although his
specimens and photographs, exhibiting the constituents of iron
and steel, were shown in Sheffield and at the Bath meeting of
the British Association. Sorby was successful in devising a
suitable technique for the preparation and examination of

•
' A. J. F. X. von Widmanstatten did not publish any account of these

experiments, which are described by Schreibers, Meteorische Stein- u. Metall-

massen, i. 20 (Vienna, 1820).

' Proc. Sheffield Lit. Phil. Soc, 1864, Feb. ; Brit. Assoc. Rep., 1864,

ji. 189.

4 METALLOGRAPHY

microscopic sections, and his later publications ' contain photo-
micrographs which have hardly been surpassed in excellence
by later workers. Sorby is certainly entitled to the credit of
being the founder of metallography, although his early obser-
vations remained almost unnoticed for twenty years, by which
time similar results had been attained by workers in other
countries.

Attempts had been made^ to examine metals by cutting
thin sections similar to those used in the study of rocks. It is
not possible to examine such sections by transmitted light,
however thin the sections may be cut, and the plan was there-
fore adopted of subjecting the sections to the attack of reagents,
so as to dissolve out certain constituents, leaving the residue
in the form of a spongy network. This method is not very
valuable, and beyond demonstrating the fact that iron or steel
containing carbon has a cellular structure, little information
was obtained by its means.

In 1878, the first communications from the Charlottenburg
Testing Laboratory appeared.^ The work of Martens is in-
dependent of that of Sorby, and has contributed very materi-
ally to the progress of the science. He was followed by
Wedding,* Stein,' and Osmond.^ All these investigators, con-
fining themselves at first to iron and steel, aimed at the
discovery of a means of controlling the quality and composi-
tion of manufactured products. The work of Osmond and
Werth was of special value in showing the ways in which
carbon, phosphorus, and other elements are distributed through
the metal, means being found, for the first time, of distinguish-
ing between intercellular and intracellular constituents. The

' Engineer, 1882, 54, 308 ; J. Iron Steel Inst., 1886, i. 140 ; 1887,
i. 255.

2 F. Osmond and J. Werth, Compt. rend., 18S5, 100, 450; Ann.
Mines, 1885, [viii.] 8, 1.

I A. Martens, Zeitsch. Ver. deut. Ing., 1878, 23, 11, 206, 480; 1880,
24, 398 ; Closer's Annalen, 1880, 7, 476 ; Stahl u. Eisen, 1882, 2, 423 ;
Verh. Ver. Bef. Gewerbefl, 1882, 233.

* H. Wedding, J. Iron Steel Inst., 1885, i. 187 ; Stahl u. Eisen, 1886,
6. 633-

' S. Stein, Stahl u. Eisen, 1888, 8, 595.

INTRODUCTION 5

arrangement of iron and steel crystals in ingots had been
determined macroscopically as early as 1868,* and the extension
of this knowledge to the microscopic structure was followed
by very numerous investigations in this direction.

The Study of Alloys

The further progress of metallography is intimately con-
nected with the study of the nature of metallic alloys. The
word alloy, or its equivalent form allay, was originally used to
signify an intimate association of two or more metals and is
so employed by Chaucer. Its special and restricted use to
denote the base metal added to gold or silver for the purpose
of working or coining, is of later origin, and is etymologically
incorrect. The present use of the word is in accordance with
its original signification.

It was long a matter of controversy whether alloys were to
be regarded as chemical compounds or as mechanical mixtures.
Perhaps the earliest researches directed towards the immediate
solution of the problem are those of Levol,''' who by the syste-
matic examination of series of alloys of progressively changing
composition, was able to show that only a few alloys remain
homogeneous throughout the process of crystallization, all
others being capable of separation into more fusible and less
fusible parts. A few alloys proved to be exceptions to this
rule, having a constant melting-point, and retaining the same
composition throughout the processes of freezing and melting.
In the alloys of silver and copper, a mixture in the proportions
represented by the formula AgjCua was found to have this
property, and was considered by Levol to be a definite com-
pound. We now know that this is incorrect, and that Level's
alloy is the eutectic mixture of the two metals, which do not
form an inter-metallic compound. Nevertheless, the research
marks a distinct advance in the knowledge of alloys.

The view that alloys are to be regarded as solidified

' D. Tschernoff, Mim. Soc. techn. Russ., April, 1868.
' A. Levol, y. Pharm. Ckim., 1850, [iii.] 17, 11 1 ; Ann. Chim. Phys.,
1852, [iii.] 36, 193; 1853, [iii.] 39, 163.

6 METALLOGRAPHY

solutions, which may or may not contain compounds accord-
ing to circumstances, is due to Matthiessen.' The method
of investigation which he adopted was the study of physical
properties, such as density, electrical conductivity, and thermo-
electric power, comparing together alloys containing varying
quantities of the same component metals, and seeking for any
discontinuous changes of properties which might mark the
presence of compounds. The work of Matthiessen was the
starting-point of the application of physical chemistry to the
study of alloys. It was followed by many similar determina-
tions of conductivity, etc.^

After the discovery of Raoult's law of the depression of
freezing-point of solutions, the study of alloys from this point
of view was suggested by the fact that alloys very frequently
melt at a lower temperature than their components. In the
same year, the depression of the vapour-pressure of mercury
produced by the addition of other metals was studied by
Ramsay,' and the depression of the freezing-point by Tam-
mann,* and by Heycock and Neville." Complete curves,
showing the change of freezing-point on passing from one end
to the other of a series of binary alloys, were published for
a number of pairs of metals by Kapp ^ and Heycock and
Neville.'

The application of the theory of phases of Gibbs ' to alloys
was suggested by Jiiptner' and by Le Chatelier." Its first

' A. Matthiessen, Brit. Assoc. Rep., 1863, 37 ; Trans. Chem. Sac,
1S67, 20, 201. Later references are given in Chapter XII.

^ G. Kamensky, Proc. Phys. Soc, 1883, 6, 53 ; Phil. Mag., 1884, [v.]
17, 270 ; V. Strouhal and C. Barus, Abh. k. bShm. Ges. Wiss., 1884, [vi.]
12, No. 14; C. Barus, Amer. J. Sci., 1888, [iii.] 36, 427. See
Chapter XII.

» W. Ramsay, Trans. Chem. Soc, 1889, 55, 521.

' G. Tammann, Zeitsch. physikal. Chem., 1889, 3, 441.

' C. T. Heycock and F. H. Neville, Trans. Chem. Soc., 1889,
65, 666.

* A. Kapp, Ann. Physik., 1901, [iv.] 6, 754.
' Phil. Trans., 1897, 189a, 25.

* See The Phase Rule, by Dr. A. Findlay, in this series.

° H. von Jiiptner von Jonstorff, Stahl u. Eisen, 1899, 19, 23.
'" H. Le Chatelier, Compt. rend., 1900, 130, 85.

INTRODUCTION 7

important application was made by Roozeboom ^ in a famous
paper, in which the results obtained in the thermal examination
of iron and steel by Roberts- Austert^ were utilized in the con-
struction of a complete diagram of the thermal equilibrium of
iron and carbon. This diagram has formed the basis of all the
subsequent discussions of the iron-carbon system, and although
it has been found necessary to modify it in a number of par-
ticulars, its general outline has been preserved in all the
schemes proposed by later workers. Roozeboom adopted the
hypothesis of the existence of three allotropic modifications of
iron, stable within different ranges of temperature, originally
propounded by Osmond,' and this hypothesis has been
generally accepted as the best expression of the known facts,
in spite of strong opposition from a school of metallurgists who
attribute the phenomena usually considered as being due to
allotropy solely to the influence of the dissolved carbon.*

The possible types of solid solutions or mixed crystals in
binary systems had been reviewed from the theoretical stand-
point of the phase rule by Roozeboom in 1899.'' The first
important application of his teaching to alloys other than those
of iron was made by Heycock and Neville in their study of the
copper-tin alloys, in which the method of quenching from deter-
mined temperatures was introduced as a method of research.'
The same paper also contains photo-micrographs which repre-
sent the highest degree of accuracy and technical perfection then
attained. The application of microscopical methods to alloys
other than those of iron was at first confined to a few alloys, such as

' H. Bakhuis Roozeboom, Zeitsch. physikal. Ckem., 1900, 34, 437;
y. Iron Steel Inst., 1900, ii. 311.

* W. C. Roberts-Austen, sth Rep- to Alloys Research Committee,
Proc. Inst. Meek. Eng., 1899, 35.

' F. Osmond, M^m. Artill. Marine, 1887, 15, 573 ; J. Iron Steel Inst.,
1890, i. 38 ; Comft. rend., i8go, 110, 242, 346.

* J. O. Arnold, y. Iron Steel Inst., 1894, i. 107 ; R. A. Hadfield,
ibid., 156, and later papers by these writers.

» Zeitsch. physikal. Chem., 1899, 80, 385, 413.

' Phil. Trans., 1902, 202a, i. This method had been employed ifl
the study of steels by H. M. Howe in 1893, Trans. Amer. Inst. Mint
Eng., 28, 466.

8 METALLOGRAPHY

the brasses and the alloys of gold,^ but its subsequent develop-
ment has been very rapid. In 1901 a number of memoirs
dealing with the structure and constitution of alloys, most of
which had previously appeared in the Bulletin de la Societe
d! Encouragement, were collected in a volume which has had a
great influence in extending the knowledge of metallographic
methods and results." In this country, the Alloys Research
Committee, under the guidance of Roberts-Austen, conducted
experiments the results of which were embodied in an impor-
tant series of reports commencing in 1891.' The work of the
committee was transferred to the National Physical Laboratory
in 1904, and is still continued.'' The school of metallographists
founded in Paris by Osmond and Le Chatelier has also con-
tributed very largely to the advancement of the study.

In the year 1903 a memoir by Tammann appeared,' in
which the investigation of the thermal behaviour of alloys was
shown to be capable of yielding very full information as to the
nature of the equilibrium of the components. Since that date
a large number of memoirs have been issued from the Gottin-
gen laboratory, and the number of binary systems investigated
has-been multiplied several times in the last few years. A few
ternary systems have also been examined. Unfortunately, the
small quantities of material used, and the insensitiveness of the
experimental method adopted, have given rise to objections,
and it is only possible to regard some of the diagrams obtained

' G. Guillemin, Compt. rend., 1892, 115, 232; G. Charpy, ibid., 1893,
116, 1131 ; 1895, 121, 494; 1896, 122, 670; F. Osmond and W. C.
Roberts-Austen, Phil. Trans., 1896, 187a, 417 ; J. O. Arnold and J.
Jefferson, Engineering, 1896, 61, 176; T. Andrews, ibid., 1898, 66, 411,
54',. 733 ; 1899, 67, 87 ; H. Le Chatelier, Bull. Soc. d^Sncourag., 1896,
[^•] Ij 559 ; J- E. Stead, J. Soc. Chem. Ind., 1897, 16, 200, 506 ; 1898,
17, 1 1 1 1 ; H. Behrens, Das mikroskopische Gefiige der Metalle u, Legierungen,
Leipzig, 1894.

^ Contribution h t Etude des Alliages, Paris, 1901.

' Proc. Inst, Meek. Eng., 1891, 543 ; 1893, 102 ; 1895, 238 ; 1897,
31 ; 1899, 35 ; I9°i> '211 (W. Campbell) ; 1904, 7 (W. Gowland).

* See H. C. H. Carpenter, R. A. Hadfield, and P. Longmuir, Proc.
Inst. Meek. Eng., 1905, 857 j H. C. H. Carpenter and C. A. Edwards,
ibid., 1907, 57 ; W. Rosenhain and F. C. H. Lantsberry, ibid., 1910,
119; W. Rosenhain and S. L. Archbutt, ibid., 1912, 319.

» Zeitsch. anorg. Chem., 1903, 37, 303.

INTRODUCTION 9

as first approximations. Tlie same method has been appHed,
but with increased experimental precautions, by Friedrich,'
Kurnakoff,^ and also by American and Italian workers.'

The further progress of metallography will be dealt with in
greater detail in the chapters allotted to its respective depart-
ments. Its growth has been so rapid as to require special
organs to serve for the collection and comparison of the results
obtained by workers in different countries. The Metallogra-
phist, established in America as an international medium in
1898, continued to serve this purpose until 1903, when it
became merged in a publication devoted to the iron and steel
industries. In the year 1904 two new periodicals appeared
simultaneously in France and Germany, namely. La Revue de
MHallurgie (monthly) and Metallurgie (fortnightly), the former
chiefly representing the school of Le Chatelier, and the latter
those of Wiist and Friedrich, both also providing a rhtime of
metallographic work published elsewhere. The work of the
Gottingen school, and much of that conducted by the Russian
investigators, appears in the Zeiischrift fur anorganische Chemie,
and that of Bancroft, Shepherd, and others in the Journal of
Physical Chemistry, whilst the Iron and Steel Institute (founded
1869), the Faraday Society (founded 1903), and the Institute
of Metals (founded 1908) also embrace metallography in their
scope. The literature of the science is, however, dispersed
through a large number of publications dealing with chemistry,
metallurgy, and engineering. The appearance, since February,
191 1, of a central organ, the Itpternationale Zeitschrift fiir
Metallographie, has proved of great advantage to the science.

The fact that metallographic researches have resulted as
yet in comparatively few far-reaching generalizations is to a
large extent due to the wide range of the systems to be
investigated. The number of metals, excluding those which

' K. Friedrich, Metallurgie, 1905, 2, No. 22, and later papers.

° N. S. Kurnakoff, Zeitsch. anorg. Ckem., 1900, 23, 439. Tlie first
publication of this and subsequent memoirs is in the J. Russ. Phys. Chem,
Soc. (in Russian).

' W. D. Bancroft, J. Physical Chem., 1899, 3, 217 ; E. S. Shepherd,
ibid., 1902, 6, 519, etc., G. Bruni, G. Sandonnini and E. Quercigh,
Zeitsch. anorg. Chem., 1910, 68, 73, etc.

to METALLOGRAPHY

are only obtainable in the laboratory with difficulty, cannot be
put at less than 30, and increases with each advance in the
knowledge of the rarer elements. The number of binary
systems which can be formed from these 30 metals is 435,
and when we consider that many of the systems are of a high
degree of complexity, owing to the presence of inter-metallic
compounds, solid solutions and allotropic modifications, it is
evident that the field is one which has hitherto been very
incompletely surveyed. The possible ternary systems com-
posed of the same metals number 4060, of which only some
half-dozen have been examined, whilst the innumerable
equilibria of a higher order remain untouched. It is not
essential that all of these possible systems should be investi-
gated, but the types of equilibrium which present themselves
are so numerous that it is unsafe to generalize as to the
behaviour of alloys except as the result of examining a very
large mass of experimental material. The requirements of
technical practice justify a very minute investigation of the
more important systems under diverse physical and mechanical
conditions. Fortunately for the science, the steels, bronzes,
brasses, and other alloys of technical importance are also
among the most interesting from a physico-chemical point of
view, and the mutual reaction of science and industry has in
this respect had the most beneficial results.

Two methods of investigation, the thermal and the
microscopical, are of primary importance in the study of
metallography. When suitably applied and combined, they
are capable of revealing the principal facts concerning the
equilibrium of the components. All other methods, although
valuable in themselves, and sometimes indispensable, must be
regarded as subsidiary to these two in the range of their
applicability. The thermal and microscopical methods will
therefore be discussed in detail, a shorter account being given
of the investigations dealing with the physical properties, such
as density, electrical conductivity, and electrolytic potential,
and with the chemical action of reagents on alloys. It will
then be shown how the experimental results are combined in
the construction of an equilibrium diagram, and how they may

INTRODUCTION *'

be made to furnish information as to the molecular condition
of the component metals. The behaviour of alloys under
mechanical stress producing deformation is another important
department of metallography, with a history of its own.
Lastly, short accounts of the metallography of the most
important technical alloys will be given as concrete illustrations
of the methods described. An appendix contains a list, with
references to the literature and brief indications of the
character of the system, of all those binary and ternary systems
of which published descriptions have been found.'

' Critical desciiptions of the systems investigated, with the equilibrium
diagrams, are provided by W. Guertler, Metallographie, Berlin, igog, and
K. Bornemann, Die bindren Metallegierungen, Halle, igog, both in course
of publication.

CHAPTER II

THE DIAGRAM OF THERMAL EQUILIBRIUM

Of the methods of metallographic investigation enumerated in
Chapter I., that which is known as thermal analysis must be
regarded as the foundation of all the others. It is rarely
possible to interpret correctly the results of miscroscopical or
other investigations without some knowledge of the diagram of
thermal equilibrium, which shows what phases may be expected
to be present in an alloy of given composition under given con-
ditions of cooling. In the great majority of cases, however, it
requires to be supplemented by microscopical examination,
whilst the magnetic, electrical, and other methods of study to
be described later all find application in special cases, and
indeed are sometimes indispensable. But the fundamental
importance of the thermal method demands for it the first
place in a work on metallography.

The basis of the diagram of thermal equilibrium is the
freezing-point curve, the co-ordinates of a point on which are
the composition of the alloy and the temperature at which
crystallization begins when the fused alloy is cooled. In the
older metallographic investigations, this was the only curve
determined,! gj^^ jjjjg jg g|.,jj jjjg ^.^gg ^jj.jj much work published
at the present day, especially in France. But the work of
Roozeboom ^ has shown that the complete thermal diagram
comprises not only the freezing-point curve, or "liquidus," but

' See, for example, H. Gautier, Bull. Soc. d' Encouragement, 1896, [v.]
1, 1293 ; C. T. Heycock and F. H. Neville, Phil. Trans., 1897, 189a, 25 ;
Trans. Chem. Soc, 1897, 71, 383.

' Zeitsch. physikal. Chem., 1899, 30, 385, 412.

12

THE DIAGRAM OF THERMAL EQUILIBRIUM 13

also curves representing the composition of the solid separating
from the fused alloy, and the transformations, if any, undergone
by the constituents after solidification. It is in fact a graphical
representation of the dependence of the number and nature of
the phases possible to the system when in equilibrium on the
composition and temperature.

When a body, such as a crucible containing an alloy, is
cooling by radiation without undergoing any change of state,
the curve connecting its temperature with the time has a
regular form, being logarithmic when the surroundings are at a
constant temperature, and rectilinear when the temperature of
the environment is progressively and regularly lowered.^ But
this regularity disappears when the cooling involves a change
of state, as in the freezing of a liquid. The continuous passage
from the liquid to the glassy or amorphous state, which
characterizes many silicates and also such substances as
sealing-wax, is not met with in alloys, which invariably exhibit
a discontinuity of properties at a definite temperature, the
freezing-point. In all cases hitherto observed, the passage
from the liquid to the solid state is accompanied by the
development of heat, although the existence of a negative heat
of fusion is sometimes heW to be theoretically possible.
The first particles of solid which separate from the solution
therefore liberate a certain quantity of heat. Any further loss
of heat by radiation, instead of reducing the temperature of
the mass, causes a further separation of solid, and this process
continues, the temperature remaining constant, until the whole
of the substance has passed from the liquid to the solid state,
after which the temperature again falls in a regular manner.
If we represent the fall in temperature of a substance cooling
without change of state by the curve A in Fig. i, the curve of
a pure substance, freezing at constant temperature, will have
the form shown at B (Fig. 2). This is an ideal curve, and
owing to undercooling, to the inequality of temperature
throughout the mass, and to other causes, the observed curves

» W. Plato, Zeitsch. physikal. Chem., 1906, 65, 721. See Chapter VI.
« G. Tammann, Krystallisierm und Schmehen (Leipzig, 1903), 35.
The possibility is, however, disputed by Roozeboom.

14

METALLOGRAPHY

deviate more or less from the form represented. The nature
of these deviations, and the means of deriving the ideal curve
from the observations, will be discussed in Chapter VI.
Neglecting these, and assuming ideal conditions, we have a
discontinuous curve, the part ab representing the cooling of
the fluid metal, the horizontal part be the process of solidifi-
cation, and the curve cd the cooling of the solid metal. The
portion cd has a steeper slope than ab, as the specific heat is
greater in the liquid than in the solid state.

Axis of Ume

Fig. I. — Cooling curve with-
out change of state.

Axis of tune

Fig. 2. — Cooling curves with change
of state.

But if, instead of a pure metal, the crucible contains an
alloy of two metals, the process of solidification follows a
different course. We will assume, as the simplest case, that
the two metals are perfectly miscible in the molten state, form-
ing a homogeneous solution, and that each of them crystaUizes
in a pure state, uncontaminated by the other, and we will
assume further that the quantity of the second metal, N,. is
small compared with that of the first, M. On coohng to a
certain temperature, crystals of M separate from the solution.
This temperature is not the freezing-point of the pure metal
M, but a somewhat lower one, as the freezing-point of a

THE DIAGRAM OF THERMAL EQUILIBRIUM 15

substance under the conditions we have assumed is lowered
by the addition of a second substance. We have now to see
in what way this lowering proceeds, as the quantity of the
second substance is progressively increased. In Fig. 3, the
vertical axis is that of temperature, the horizontal axis repre-
sents the composition of successive mixtures. It is usual to
express this composition as a percentage. We then represent
the proportion of the second metal present by figures from o
to 100. At the origin of the axes we have the pure metal M ;
we therefore say that the concentration of the metal N at that
point is zero. At the right-hand limit of the diagram we have
the pure metal N, the concentration of which is then 100 per
cent. It is often convenient, especially when deaUng with the
theoretical interpretation of the diagram, to express the com-
position of the alloy in terms of atoms or molecules. In that
case, the points on the axis of abscissae represent the atomic or
molecular concentration, that is, the number of atoms or mole-
cules of the second metal present in 100 atoms or molecules
of the mixture. Atomic concentrations will be g«ierally
employed in the sequel. The concentration of M is of course
found in all cases by subtracting that of N from 100.

Neglecting for the present the remainder of the cooling curve,
and considering only the temperature at which crystallization
commences, the effect of the addition of a small quantity of
the metal N is to depress this temperature from m (the freezing-
point of pure M) to ni (Fig. 3). Further successive small
additions of N lower the initial temperature of crystallization
still more, for example, to ni' and «'". In exactly the same
way, if n be the freezing-point of the pure metal N, successive
small additions of M lower the temperature at which solidifi-
cation begins successively to «', «", and «"'. If in Fig. 3,
therefore, the abscissae are taken to represent the proportions
of M and N in the fluid mixtures and the ordinates the
temperatures at which the first particles of solid separate on
cooling, these temperatures lie on two curves (represented for
the sake of simplicity as straight lines) sloping downwards from
m and n respectively. A point, e, must consequently exist at
which these two curves intersect. The alloy corresponding

i6 METALLOGRAPHY

with the composition c, found by dropping a perpendicular
from e upon the concentration axis, has the lowest initial
freezing-point of the whole series. It is hence known as the
eutectic alloy (Greek evTrjKTO's, easily-melting, from eJ + tj/k-civ)
or simply eutectic, and the point e is called the eutectic point,
the introduction of these convenient terms being due to
Guthrie.

Returning now to the cooling curves of individual alloys,
represented in Fig. 2, the point at which the curve changes its
direction, owing to the development of heat on solidification,
is lowered by the addition of the second metal from m to tri.
The portion of the curve which represents the passage from
the liquid to the solid state, however, is no longer horizontal,
as be; that is, the soUdification of the mass no longer takes
place at constant temperature. For the separation of the first
crystals of the metal M alters the composition of the part
remaining fluid, which is now richer in N than before.
Referring to Fig. 3, it will be seen that the separation of
crystals from such a mixture takes place at a temperature
lower than m'. Consequently, as the metal M is withdrawn
from the molten alloy by crystallization, the mother-liquor
becomes progressively richer in N, and the temperature at
which it can deposit more crystals progressively sinks. This
gives to the part Vd of the cooling curve C (Fig. 2) the sloping
form indicated. For alloys containing more and more of the
second metal N, the slope of the cooling curve becomes
steeper and steeper, as in D. The change in direction at the
point U' or V" being much less than at b, the temperature of
initial solidification becomes more difficult to determine as the
proportion of N in the alloys increases.

In all these cases, as M is withdrawn from the molten mass,
a point must be reached at which the mother-liquor has the
composition and temperature represented by the eutectic point
e. In order to determine what happens at this point, it will
be convenient to consider the freezing-point curve (Fig. 3)
from a somewhat different standpoint. The left-hand branch
of the curve, me, may be considered as a solubility curve, since
it represents the temperatures at which solutions of M in N

THE DIAGRAM OF THERMAL EQUILIBRIUM 17

become saturated with respect to M. In like manner, the
right-hand branch represents the temperatures at which solu-
tions of N in M (for in alloys either metal may be regarded
in turn as solvent or as solute) become saturated with respect
to N. Now, at the eutectic point e, being the point of inter-
section' of the two solubility curves, the solution is simul-
taneously saturated with M and N. Should c^stals of M
separate, it at once becomes supersaturated with respect to N,
and equilibrium can only be restored by the separation of
crystals of N. The metals M and N therefore crystallize

Cov^cervtraxioTi,

Fig. 3.

together, the temperature remaining constant until the whole
of the mass has solidified. This constancy of freezing-point
is characteristic of eutectic mixtures.

In an alloy with the initial freezing-point »*', containing
only a small quantity of the metal N, the greater part will
have solidified before the eutectic point is reached. The
amount which will solidify at the eutectic temperature is
therefore very small, and will be represented by a very short
horizontal portion of the cooling curve, as at e'f in the curve C
(Fig. 2). As alloys richer in N are examined, this horizontal
portion becomes more strongly marked, as in D. Finally, an

T.P.C. ''

i8 METALLOGRAPHY

alloy having exactly the eutectic composition solidifies com-
pletely at constant temperature, so that its cooling curve has
the form E. When the proportion of N in the alloys is further
increased, so that the crystals which separate first are those of
the metal N, the length of the eutectic horizontal again
decreases, becoming less and less as the composition of the
alloy approsfthes the pure metal N.

We are now in a position to construct the complete diagram
of thermal equilibrium for alloys of M and N. The time taken
for the eutectic mother-liquor to solidify, that is, the length of
the lower horizontal portion of the cooling curve, may be con-
sidered as proportional to the quantity of eutectic present, the
conditions of cooling of all the alloys being assumed to be
identical. These " eutectic times " may be plotted as ordinates
against the composition of the alloys as abscissae, as in the
lower part of Fig. 4. The intersection of the two branches
gives the eutectic composition, since it indicates the maximum
time of eutectic solidification. When the two branches of the
freezing-point curve are strongly curved instead of being, as we
have represented them, straight lines, it may be difficult to fix
their exact point of intersection, and the plotting of the eutectic
times then becomes a valuable auxiliary in fixing the position
of the eutectic point. We owe this use of time-composition
curves, which finds much wider application in the more com-
plex cases to be considered later, to Tammann.'

The exact position of the two branches of the freezing-point
curve now having been determined, the remainder of the
diagram may be constructed. Since the cooling curves of all
mixtures of M and N show an arrest at a constant temperature,
the eutectic temperature, a line may be drawn across the
diagram from C to D (Fig. 4, upper part) through the eutectic
point E, and parallel with the axis of concentration. This line
represents the solidification of the eutectic mother-liquor, and
is called the eutectic horizontal. A vertical line from E to F
separates those alloys which contain an excess of the metal M
over the eutectic proportion from those which contain an excess
of N.

' ZeiUch. anorg. Chem., 1903, 37, 303 ; 1905, 45, 24 ; 1905, 47, 289.

THE DIAGRAM OF THERMAL EQUILIBRIUM 19

Assuming that the alloys are not heated to so high a tempe-
rature as to produce an appreciable amount of metallic vapour,
any alloy the temperature and composition of which is repre-
sented by a point lying above the freezing-point curve AEB is
in a liquid state. Points on AE or EB represent the com-

c

M

N. Liguici

M + eutectic

F

N + eutectie

CoTicencraCit

1TV

B

N

ConcentrcLtCoTv

Fig. 4. — Thermal analysis, simplest case.

mencement of crystaUization of the metals M and N respec-
tively. An alloy represented by a point within the triangle ACE
consists of crystals of M, together with a still Uquid moiher-
liquor. In the same way the triangle BDE encloses mixtures
of N with mother-liquor. All alloys below the line CD are
solid. If to the left of the line EF, they consist of crystals of
M together with eutectic j if to the right, of crystals of
N -1- eutectic.

20 METALLOGRAPHY

The Form of the Freezing-point Curve

In the simplest case, the lowering of the freezing-point of a
metal by the addition of a second metal is proportional to the
number of molecules of the latter added. This is the well-
known law of Raoult, which may be expressed by saying that
on the addition of one molecule of N to loo molecules of M
the freezing-point is depressed by an amount independent of
the nature of the added metal, and known as the molecular
depression. Two assumptions are here made, as in the earlier
part of this chapter, namely, (i) that the two metals do not
form a compound under the conditions of the experiment, and
(2) that the metal M crystallizes from the fluid alloy in a pure
state. It will be shown later that the metals are frequently
monatomic, but it is best for the present to make no assump-
tion as to their molecular complexity, and therefore to use
atoms instead of molecules as the units. The composition of
an alloy is then expressed in atomic percentages, the abscissae
of the temperature-concentration diagram being the number
of atoms of one of the component metals in 100 atoms of the
alloy. The atomic percentage may be calculated from the
percentage by weight in the following manner : —

If/ and (100 — /) are the percentages of the metals M and
N respectively, a and b their atomic weights, and x and {loo—x)
the atomic percentages in the mixture, then

X = ^

p -t-(ioo -/)|

/ , 100(100 — *)
(100 — *) = ' ^'

Pj+i^oo-p)

Since the calculation of the atomic proportions of each
alloy examined is very tedious, it is advisable to make the
calculation for three or four percentages only, and then to' plot
the atomic percentages found against the percentages by weight,
to draw a smooth curve through the points, and to find the'

THE DIAGRAM OF THERMAL EQUILIBRIUM 21

composition of the remaining alloys of the series by graphical
interpolation.

An important advantage of the use of atomic percentages
instead of percentages by weight is that the branches of the
freezing-point curve are then straight lines for the range over
which the laws of dilute solutions hold good, which is generally
the case up to 5 or 10 atomic per cent. The slope of the
curve, dx/dt, then represents the depression of freezing-point
produced by the addition of one atom of the second metal.
This is called by Heycock and Neville the atomic fall. The
deviations from the rectilinear form of such curves, and the
mode of calculation of the molecular complexity of the dis-
solved metal, will be discussed later, in connection with the
study of the exact form of freezing-point curves, and the
formulee which have been employed to represent them.^
The most conspicuous advantages of the atomic method of
plotting, however, will be seen in dealing with inter-metallic
compounds.

The Eutectic Alloy

The fact that with many pairs of metals it is possible to
prepare one alloy which has a freezing-point lower than that of
any other member of the series has long been known, and was
made use of in the preparation of the so-called " fusible metals."
The production of a liquid by mixing a solid salt with solid
ice in " freezing mixtures " was also well known.^ As far back
as 1864, Riidorff' gave the correct explanation of the produc-
tion of freezing mixtures, showing that the point of minimum
temperature thus obtained was the intersection of the curve of
separation of ice from salt solutions with that of the solubility
of salt in water. The fact that metals showed a similar
behaviour, so that on mixing two solid amalgams a liquid

1 See Chapter XV.

" For the earlier history of freezing mixtures, see C. G. von Wirkner,
Geschichte und Theorie der Kdlteerzeugung, Hamburg, 1897. The earliest
■quantitative measurements are those of R. A. F. de Reaumur, Man. Acad.
■Set., 1734-

» Fegi^. Ann., 1864, [v.] 2, J37.

22 METALLOGRAPHY

amalgam might be produced with considerable lowering of
temperature, in complete analogy with freezing mixtures, had
been observed by Dobereiner in 1824.^

The freezing-points of a very extensive series of salt solu-
tions were investigated from this point of view by Guthrie.
Unfortunately, however, in spite of the work of RiidorfF, men-
tioned above, and of de Coppet,^ the constancy of composition
and freezing-point and the characteristic appearance of the
mixture of minimum freezing-point led Guthrie to regard it as
a combination of the salt with water, stable only below 0°, to
which he gave the name of cryohydrate. In his later investiga-
tions^ the complete resemblance between the behaviour of
cooled salt solutions, alloys, and mixtures of fused salts was
shown, and the word " eutectic " was introduced.* The view
that the cryohydrate was a chemical compound had been com-
bated in the meantime by Pfaundler,° but was long popular.
From the fact that cryohydrates always contain a large quantity
of water in proportion to that of the salt, it is usually possible
to find a formula of a hydrate to represent its composition
approximately. In alloys and mixtures of fused salts in which
the constituents are present in moje nearly equal proportions,
the deviation from a simple molecular ratio is often very
marked, and the composite nature of eutectics was conse-
quently more readily admitted. Even here the occasional
approach of eutectic alloys to simple formulae led to their being
regarded as compounds ; and even at the present time the
assignment of a formula to the alloy occupying a minimum
on the freezing-point curve is not unusual in some published
work. A typical case is that of the eutectic alloy of copper
and silver, which was long known as Levol's alloy ° and
regarded as a compound having the formula Ag3Cu2.

' Schweigg. J., 1824, 48, 182.

"^ F. Guthrie, Phil. Mag., 1875, [iv.] 49, I, 206, 266 ; 1876, [v.] 1, 49,
354, 446 ; 1876, [v.] a, 211 ; 1878, [v.] 6, 35, 105.

' Bull. Soc. VaudoiseSci. Nat., 1871, [ii.] 11, I.

* Phil. Mag., 1884, [v.] 17, 462 ; Proc. Phys. Soc, 1884-5, 6, 124, 169.

' Ber., 1877, 10, 2223 ; see also Offer, Sitzutigsber. Wien. Akad. Set,,
1S80, 81, 1058.

' A Levol, y. Pharm. Chim., 1850, [iii.] 17, in.

THE DIAGRAM OF THERMAL EQUILIBRIUM 23

From what has been said above, in discussing the thermal
diagram, it will be seen that the eutectic is really a conglomerate
of the two components. From the fact that its cooling curve
has the same form as that of a pure substance, namely, that of
Fig..2,B, and that the crystallization of both components takes
place simultaneously, so causing a very intimate mixture, the
mistake of considering it as homogerieous is readily explained.
It will be seen later, in treating of the microscopic and other
properties of alloys, that the appearance of eutectic alloys is
characteristic, and often quite unlike that of mere mechanical
mixtures. The heterogeneity, of cryohydrates was not definitely
proved until 1895, whenPonsot^ showed by microscopic
examination that distinct crystals of ice, and of potassium
permanganate, potassium dichromate, and copper sulphate,
could be observed in the frozen cryohydrates of those salts.
The same thing was shown for the cryohydrates of colourless
salts by examination in polarized light. Considering the
false idea implied by the term " cryohydrate," it is better to
abandon its use, and to employ the term " eutectic " for salt
solutions, alloys, mixtures of fused salts or organic substances,
and igneous rocks alike.

Discussion of the Phase Equilibrium

The presentation of metallographic results in the language
of the phase-doctrine is so frequent, that it is desirable to
discuss the simple case already examined, from this point of
view. It is outside the scope of this work to set forth the
principles of the phase rule, which have been fully explained
in another work of this series.'^ Here it will be sufficient to
take the rule itself for granted, referring elsewhere for its
justification.

In all the cases to be considered in metallography, the
components of the system are the pure metals. The phases are

> Bull. Soc. Chim., 1895, [iii.] IS, 3t2.

• The Phase Rule, by Dr. A. Findlay. For the theoretical basis on
which the rule is founded, see the work by Dr. F. Donnan, on Thernw
dynamics, in the same series.

i4 METALLOGRAPHY

such homogeneous portions of the system as are separated
from each other in space by bounding surfaces. Thus the
vapour forms a single phase. There may be one or more
liquid phases, for instance, a vessel containing mercury, water,
and oil, shows three coexistent liquid phases, separated from
one another by definite bounding surfaces. Amongst alloys,
the system lead-zinc affords an example of two separate
liquid phases. The various types of crystals which may be
present in solid alloys also constitute distinct phases. The
number of degrees of freedom of a system is "the number of
the variable factors, temperature, pressure, and concentration
of components, which must be arbitrarily fixed in order that
the condition of the system may be perfectly defined." ^

The phase rule now states that, if p be the number of
phases, n the number of components, and / the number of
degrees of freedom, then

/= n-p+2
when the system is in equilibrium. Consequently, for a.
system of a given number of components, the greater the
number of phases present, the less is the variability.

A certain simplification may be introduced into most of
the cases with which we have to deal in metallography. It is
commonly permissible to neglect the vapour phase when con-
structing the freezing-point diagram, the volatility of most
metals at their melting-points being small. Cases in which
the vapour phase is important form a separate class. In the
same way, the equilibrium may be assumed to be reached
under a constant pressure, that of the atmosphere, since the
vessels in which the fusion is carried out are either open to
the air or communicate with vessels in which the pressure is
that of the atmosphere. Pressure may therefore be omitted
from the variables to be considered. The influence of pressure
on freezing-point, and on the equilibrium of systems of two or
more components, is of great scientific interest, but may be
neglected in all but exceptional cases when dealing with
metallic alloys. In the closely related subject of the formation

' Find lay. Phase Rule, p. 15.

THE DIAGRAM OF THERMAL EQUILIBRIUM 2S

of igneous rocks, howeverj this influence becomes a factor of
the very highest importance.

The eifect of omitting all consideration of the vapour
phase and of changes of pressure from the study of alloys is to
reduce the number of variables to two, namely, temperature
and concentration. The conditions of equilibrium are then
represented by the reduced formula

and it is in this form that the phase rule is most usefully
employed in the consideration of alloys.

Applying this rule to the case represented in Fig. 4, it will
be seen that the whole area above the curve AEB represents
mixtures of the two components containing only a single phase,
the liquid, so that

/' = 2 - I + T = 2

the system has, therefore, two degrees of freedom, or is said to
be bivariant. This means that both temperature and con-
centration may vary independently, without any alteration of
the number of phases. Let the temperature now change so
that a point on the curve AE or EB is reached. Solid begins
to separate, that is, a new phase appears. There being now
two phases, liquid and solid,

/' = 2 - 2 + I = I

the number of degrees of freedom is reduced to one, and the
system is said to be univariant. In this case, any change of
temperature determines a corresponding change in the com-
position of the mother-liquor or liquid phase. Cooling the
alloy causes a further deposition of the solid phase, which
being in this case the pure metal, does not vary in composi-
tion, and the part remaining liquid is correspondingly im-
poverished. For points on the curve AEB, therefore, to
every definite temperature corresponds a definite composition,
and fixing either temperature or concentration immediately
fixes the other variable.

At the eutectic point E, the two solid metals are simul-
taneously in equilibrium with the liquid. There are. thus three

26 METALLOGRAPHY

phases present, and the number of degrees of freedom is
therefore reduced to zero, or the equilibrium is only possible
at a definite temperature and concentration, the eutectic
temperature and eutectic concentration. The system is now
said to be invariant. The two solid phases are deposited in
a constant proportion, so that the composition of the mother^
liquor, as well as its temperature, remains constant during the
whole process of solidification.

It follows from this that the eutectic alloy is not to be
regarded as a phase, but as an intimate mechanical mixture of
two solid phases, in this case the two component metals. In
the diagram. Fig. 4, therefore, the whole of the area CDNM
represents mixtures of the two solid phases, and the line EF
does not mark a boundary between distinct phases. From a
micrographic point of view, however, it is necessary to make
a distinction between the metal which has solidified as primary
crystals along the line AE or EB, and that which has solidified,
in intimate association with the second metal, at the point E.
This is effected by treating the pure metals M and N and the
■eutectic alloy as three separate micrographic constituents,
•although the former are single phases, and the latter a con-
glomerate of two phases. The area CEFM then represents
mixtures of primary crystals of M with the eutectic, and EDNF
mixtures of primary crystals of N with the eutectic, and this
•distribution of phases is indicated in the diagram.

The relative proportions of the several phases present in
any alloy of any given composition at a given temperature are
readily determined from the diagram. In Fig. 5, the upper
J)art of Fig. 4 is repeated. The point g represents an alloy of
which a part is still liquid, the remainder having solidified in
the form of crystals of the pure metal M. It is required to find
the proportion of solid in the mixture, and the composition
of the still liquid portion. A horizontal line drawn through g
cuts the freezing-point curve EB at r, and the vertical axis at S.
The solid phase being, by hypothesis, the pure metal, its
composition is represented by S. The composition of the
liquid phase is given by the point r, from which the percentage of
the metal M in the liquid is found by dropping a perpendicular

THE DIAGRAM OF THERMAL EQUILIBRIUM 27

on to the concentration axis at x. The alloy represented
by the point g therefore consists of crystals of the metal M
and a liquid portion which contains Ma- per cent, of N and N^
per cent, of M.

If the temperature and concentration of the original
liquid alloy were represented at a particular moment by the
point 0, then on cooling, the curve AE is cut at the
point /. At this temperature, the composition of the liquid
phase is unchanged. As crystals of M separate, however,

>v

F ^

--'- - -

K

< /

9

\./

h

X,

B

S
C

M

■' Fig. s.

the point representing the concentration of the two metals
in the liquid phase moves along the line AE from / to
r. A simple geometrical consideration shows that the solid
and liquid phases are now respectively present in the ratio of
gr to S^. Since the liquid phase must ultimately reach the
eutectic concentration, represented by the point E, the relative
proportions of solid and liquid respectively on reaching the
eutectic temperature are given by the ratio ^E : C/4. Since the
eutectic solidities as a distinct micrographic constituent, it is
convenient for many purposes to express the composition of
the completely solidified alloy in terms of the percentage of
crystals of the free metal M or N and of the eutectic. For
this purpose, a simple graphical method introduced by

28

MET A LLOGRA PHY

Sauveur' is very convenient. The vertical axis (Fig. 6) is
divided into loo parts, and the horizontal axis represents the
percentage of the two metals in the alloy. The eutectic com-
position being as before E, straight lines are drawn from M
to E and from E to N. An alloy of the composition x, that
is, containing M* per cent, of the metal N and Njc per cent,
of M, will in the solid state be made up of xy per cent, of

Crystals of
N

Fig. 6. — ^^Micrographic constituents of a simple binary system.

eutectic and yz per cent, of crystals of M. This form of
graphical construction will be frequently employed.

The Melting-point

We may now consider what happens when a solid alloy of
this type is heated. If the alloy has exactly the eutectic
composition, the whole of it will liquefy at the eutectic tem-
perature, although this is much below the melting-point of
either of the component metals. If, on the other hand, it
consists of a mixture of crystals of one of the metals with the
eutectic, liquefaction of the eutectic portion only will take place
at this temperature. The temperature*will then rise, the now
fluid eutectic acting as a solvent for the solid crystals, and the
composition of the liquid changing in a manner which may be
represented by a point travelling upwards along the solubility
curve, until the last crystals are dissolved at a temperature

' Mttallographist, 1898, 1, 27.

THE DIAGRAM OF THERMAL EQUILIBRIUM 29

which is the same as that at which crystallization first began
on cooling. The course of events on melting is in fact the
reverse of that on freezing, and the heating curve is the
reverse of the cooling curve, if we suppose the rate of influx of
heat to be sufficient to maintain equilibrium throughout the
process. In practice, however, it is far more difficult to ensure
equilibrium during melting than during freezing, and heating
curves are therefore rarely employed at the melting-point
except as an occasional mode of controlling cooling curves.

Since the eutectic alloy is only a conglomerate of two
phases, it would seem that a sufficiently intimate mixture of
the two metals should melt at the eutectic temperature, and
such is found to be the case. With mixtures of coarse particles,
such as filings, the contact is not sufficiently intimate, and
melting does not take place to an appreciable extent until a
somewhat higher temperature has been reached. This point
has been investigated experimentally m the case of mixtures
of lead and tin.' The eutectic alloy of this series melts at
180", and a mixture of the finely powdered metals, the particles
of which did not exceed 0T5 mm. in diameter, melted exactly
at that temperature when slowly and regularly heated. When,
however, the particles varied in diameter from o'ls to o"S2 mm.,
the melting-point rose to 183°, and coarser particles required
a still higher temperature to produce fusion. A similar pheno-
menon, of some practical importance, occurs in the case of
grey cast iron.* The graphite, which is one of the constituents
of the eutectic, collects together during solidification in the
form of comparatively coarse plates. These plates are only
slowly dissolved on heating by the metal in immediate contact
with them, with the result that the melting-point may be as
much as 70° higher than the freezing-point.

Compression of the particles will evidently increase the
intimacy of contact, and a compressed mixture of metals is
therefore more likely to melt at the eutectic temperature than
one mixed by stirring or shaking only. The fact that fusible
metals, such as Wood's and Rose's alloys, which melt in hot
> C. Benedicks and R. Arpi, Metallurgie, 1907, 4, 416.
« P. Goerens, ibid., 137.

30

METALLOGRAPHY

water, can be formed by subjecting their components to a
pressure of 7300 atm.,' does not prove, as was at first supposed,
that a reaction occurs between the metals in the solid state, as
the result may be explained as being due to the bringing of
the particles into such close contact by pressure that equi-
librium is readily attained on heating to the eutectic
temperature.

°c

liOO

tooo

900

''

■^v^ Eutectic times ^,''' /

300

/

100

/

600

/

SOO

Liquid. /

4^

/ Ail + liquid'

300

\

/

too

7Z.^>>s^^

100

n

feutecUo

Au ■<■ eutectic

Fig. 7. — Thallium and gold.

Two typical diagrams for eutectiferous series of alloys are
shown in Figs. 7 and 8. Fig. 7 represents the alloys of gold'
and thallium'' which do not form either mixed crystals or
compounds. The two branches of the curve are very nearly
straight lines, indicating a very simple constitution of the
solution; The duration of the eutectic arrest on the cooling
curves is plotted at the top of the diagram in an inverted form
for convenience, and it will be seen that the eutectic composi-
tion is sharply determined by its means.

1 W. Spring, Bfr., 1882, 15, 595.

' M. Levin, Zeiisck. anorg. Ckem., 1905, 45, 31

THE DIAGRAM OF THERMAL EQUILIBRIUM 31

. The second example is that of the alloys of silver and lead^
the equilibrium diagram of which is represented in Fig. 8.'-
The form of the freezing-point curve in this case is of great
practical importance, since on it depends the well-known
Pattinson process for the desilverization of lead. The eutectic
point lies, as will be seen from the diagram, very near to the
lead end of the curve, the eutectic alloy containing only 4
atomic per cent, of silver, or 2-25 per cent, by weight. The

Ag + eutectic

to io 30 49 &0 60 70

T

%Ay iy

Y Acom~%Aff

Fig. 8. — Lead and silver.

silver branch of the curve is far from being straight, and has a
point of inflection, the appearance is therefore different from
that of the gold-thallium diagram, but the arrangement of the
fields of phase-equilibrium is nevertheless the same. It is not
certain whether the eutectic horizontal reaches exactly to the
limits of the diagram, as assumed, but microscopical examination

' The upper branches of the curve were determined very accurately by
Heycock and Neville (/%?'/. Trans., 1897, 189a, 25). The complefse
diagram is drawn from the observations of K. Friedrich (Metallurgie, 1906,
3, 396) and G. J. Petrenko (Zeitsth. anorg. Chem., 1907, 53, 200). All
three sets of figures are in good agreement.

32 METALLOGRAPHy

and the study of the cooling curves indicate, that this must be
very nearly true.'

The object of the Pattinson process is the fractional
crystallization of an extremely dilute solution of silver in
molten lead. The lead, containing ccoog per cent of silver or
more, is cooled until a certain propoirtion — for instance, seven-
eighths — has crystallized, and the crystals are removed by means
of a perforated ladle. The portion remaining liquid is then
nearer to the eutectic composition than the original alloy,
that is to say, it is richer in silver. The crystals collected are
those separating on the left-land branch of the curve in Fig. 8,
and consist of practically pure lead. Since, however, a portion
of the mother-liquor is always retained mechanically, it is
necessary to re-melt the crystals, and to repeat the process.
The liquid portions are also again partially frozen, until the final
products are almost pure lead, containing at most o'ooi per
cent. Ag, and an alloy of the eutectic composition (2*25 per
cent. Ag by weight, or 4 atomic per cent.). Since this alloy
solidifies at constant temperature, it is evident that the process
of fractionation cannot bft carried any further.

Case in which a Compound of the Two Metals may
Separate from the Molten Alloy

The next case to be considered is that in which the two
metals can combine together to form a definite compound.
The existence of such definite inter-metallic compounds has
now been placed beyond doubt, although the exact determina-
tion of their formulas is one of the most troublesome problems
of metallography, and all attempts to bring them into line with
the conceptions of valency which prevail in chemistry have so
far proved unsuccessful.'' The properties of alloys are pro-
foundly modified by the presence of such compounds, and
the determination of the limits of their existence in each case

' The account of the Pattinson process, given in Abegg's Handbuck der
anorganischen Cheniie (vol. ii. part i. p. 668), incorrectly assumes the
formation of solid solutions of silver and lead.

» See Chapter XT.

THE DIAGRAM OF THSJiMAL EQUILIBRIUM 33

becomes of great importance in the complete metallographic
study of any series of alloys.

It is probable that the methods employed in metallography
are not capable of detecting all the possible inter-metallic
compounds. The reason for this is that experiments with
alloys are commonly made under atmospheric pressure, at
which many compounds of volatile metals may be more or
less completely dissociated at their melting-point, and further
that heating to a temperature far above the melting-point may
be sometimes necessary to effect combination. As Tammann
has remarked, a mixture of liquid oxygen and liquid hydrogen
would, if frozen, give no indication of the existence of a com-
pound of the two elements, whereas the result would be very
different if the oxygen and hydrogen had been previously
heated to such a temperature that water was formed. Metals,
which frequently react with one another very sluggishly, may
co-exist in a state of false equilibrium, which may be difficult
to distinguish from one of true equilibrium. A case of this
kind has been met with in alloys of aluminium and antimony.'
These two metals only combine very slowly when heated
together in the liquid state, so that, in a given experiment,
only one-tenth of the total mass had combined after heating
equivalent proportions at 715° for 100 minutes, three-quarters
combining after heating for 30 minutes at 1 100°.

In the present chapter, it will be assumed as before that
the alloys are throughout in a state of equilibrium, and that
the two component metals and the inter-metallic compound
separate from the liquid in a pure state, that is, that solid
solutions are not formed. Two cases may occur.

A. — The Freezing-point Curve shows a Maximum

When the compound formed by the union of the two metals
is so stable that it may be heated to its melting-point without
decomposition, it behaves similarly to a pure metal, and its
freezing-point is depressed by the addition of either of the

> G. Tammann, Zeilsch. anorg. Chem , 1905, 48, 53.
T.P.C. D

34 METALLOGRAPHY

components. The freezing-point curve therefore shows a
maximum corresponding with the composition of the com-
pound. If the affinity of the one metal for the other be so
great that the compound is formed with a large development
of heat, this maximum may lie considerably above the freezing-
point of either of the component metals. A conspicuous
example of this is found in the amalgams of the alkali metals
with mercury. Whilst the freezing-points of mercury, sodium,
and potassium are respectively — 38-8°, 97-6°, and 62-5°, the
compounds NaHga and KHga solidify at 346° and 279° respec-
tively.'

A compound such as NaHga, occupying a maximum on the
freezing-point curve, melts to a liquid of the same composition
as the solid. Fusion and solidification therefore take place at
a constant temperature, and the cooling curve of the compound
is in every respect like that of a pure metal. This being so,
alloys containing as solid phases only the compound MN and
one of its component metals, for instance M, may be con-
sidered as a binary system of the same type as those already
discussed, consisting, that is to say, of two branches inter-
secting in a eutectic point. A second binary system is
made up of alloys containing MN and N as solid phases.
Two diagrams similar to Fig. 4 might be placed beside one
another, the second descending branch starting from the
point B. This would, however, give a sharp point to the
summit representing the compound, and this is incompatible
with equilibrium, the conditions of which demand that the
tangent to the curve at such a point shall be horizontal. The
flattening of the curve at the summit indicates that the com-
pound is dissociated to some extent into its components in the
liquid phase, the flatness of the curve increasing with increasing
dissociation.

The form of curve which presents itself in practice is best
illustrated by a concrete example. Fig. 9 has been con-
structed from the results obtained by two different observers

' Schiiller, Zeitsch. anorg. Chem., 1904, 40, 385 ; N. S. KurnakofF,
ibid., 1900, 28, 439 ; E. Janecke, Zeitsch. physikal. Chem., 1907, 58,
245.

THE DIAGRAM OF THERMAL EQUILIBRIUM 35

for the alloys of magnesium and tin.^ It will be seen that the
curve rises to a very pronounced maximum at 667 atomic per
cent. Mg, indicating the formation of a stable compound
MgaSn, melting at 783-4°, that is, considerably above the
melting-point of either of the component metals. Two eutectic
points occur, the solid phases Sn and MgjSn being in equilibrium
with the liquid phase at 2 1 0°, and the phases MgjSn and Mg
similarly at 565°. Since the eutectic arrests, as shown by the

''C ID 20 30 to SO sa eg lO O % Mg

- iy weight

+Ugiai

Sn- +ligiuel'

Sn, + M^2 ^^

U^gSrv-^rMff

zo

«»

too
Atom % Mn

Fig. 9. — Tin and magnesium.

dotted curves, only vanish at the limits of the diagram and at
the composition of the compound, it is evident that the con-
ditions assumed in this chapter are fulfilled, namely, that the
solid components separate from the liquid in a pure state.
The middle branch of the curve is considerably rounded at
the summit, indicating that in the fused compound a certain
amount of dissociation takes place, represented by the equa-
tion—

Mg^Sn^zMg + Sn

' G. Grube, Zeilsck. anorg. Chem., 1905, 46, 76; N. S. KurnakoflF,
ibid., 177.

36

METALLOGRAPHY

It will be seen that at all temperatures between 565"
and 783°, there exist two liquid alloys with which solid
MgaSn can be in equilibrium, the one containing an excess
of tin, the other an excess of magnesium. The constituents
present in solidified alloys of this series are represented in
Fig. 10. On account of the great difference in atomic weight

Percentage of My by weight,

Atomic percervULge' of Mg

Fig. 10.

between the two metals, the atomic percentages used in
plotting differ greatly from the percentages by weight, which
are added for comparison above Fig. 10.

B. — The Freezing-point Curve shows a Break, but no
Maximum

Many inter-metallic compounds break up below their

melting-point into a liquid alloy and crystals of another

compound. This case is comparable with that of many

hydrated salts, which dissociate on heating into an aqueous

solution and a hydrate containing less water. The hepta-

hydrate of zinc sulphate, ZnSOijyHaO, for instance, is stable

at ordinary temperatures, but if heated to 39°, it decomposes,

and at this temperature there is equilibrium between the

liquid and vapour phases and two solid phases, namely, the

heptahydrate and the hexahydrate, the latter being the stable

form above 39° :

39°
ZnS04,7H20-> ZnS04,6H20 -|- sat. solution

THE DIAGRAM OF THERMAL EQUILIBRIUM 37

There being two components and four phases, the system is
invariant, and any increase or decrease of temperature must
cause the disappearance of one of the solid phases.

Similar cases are common amongst metallic alloys. The
conditions are represented in Fig. 11, The pure metal M
separates along the branch AE, the eutectic point being
reached at E. The ascending branch EF corresponds with
the crystallization of a compound of the two metals M and N,
but instead of reaching a maximum and again falUng to a
second eutectic point, as in the case of the alloys of mag-

iSic

y/"^ ^ +

/^ V^ui^

A /^ ^

MN3+- N

liqvxtt X. /

D

""'''•-* ^ .-— """"^

M + MN3

:> K

20 4-0 eo ao

Atomic percentage of N

Fig. II.

nesium and tin, the freezing-point curve changes in direction
at the point F, and ascends to the temperature of solidification
of the pure metal N at B. The branch BF then corresponds
with the crystallization of N from the molten alloys. The
break at F is to be interpreted as follows. The compound
dissociates at the point F, partly melting to a liquid, and
giving up the whole of the metal M contained in it to the
liquid, leaving crystals of N. At the temperature represented
by F, then, there is equilibrium between two solid and one
liquid phase :

^:^y~^ N + (solution of N in M)

(0

38 METALLOGRAPHY

Neglecting the vapour phase, we have three phases and two
components, or, since f^ = n — p + i, we have fi = o, or the
system is invariant, and can only exist at a definite temperature
and concentration of the liquid phase.

It now remains to determine the formula of the compound
separating along the branch EF. In many of the older
determinations, the composition at the point F was taken to
be that of the compound. But it is easy to see that this is by
no means necessarily the case, although it is one of the possible
conditions, and does occasionally occur. The point F is to be
regarded as the intersection of two solubility curves, and may
frequently lie below the maximum of EF. We may, in imagi-
nation, continue the curve EF until it reaches a maximum at
G, and regard the curve FGH as that which would be followed
by the compound in crystallizing were it not for its dis-
sociation, but such extrapolation is difficult and uncertain. A
better means is afforded by the study of the cooling curves.
Alloys having a composition between F and the pure metal N
will show a development of heat at the temperature F, due to
the reaction between a part of the N crystals which have already
separated and the still liquid alloy, to form the compound, that
is, the reaction represented by the lower arrow in equation (i).
The maximum development of heat owing to this reaction co-
incides with the composition of the compound, as indicated by
the arrest curve F/K. Additional evidence is afforded by the
form of the curve C«D. The arrest due to the solidification
of the eutectic E becomes smaller after the eutectic com-
position is passed, finally vanishing when the compound is
reached at D. In Fig. ii, the curve F/'K reaches its maxi-
mum, and the curve C^D ends, at 75 atomic per cent, of N, that
iSj the compound has the formula MNj.

The amount of each constituent present in the solidified
alloys can be determined from Fig. 12. This is, however, an
ideal case which presupposess slow cooling, so that complete
equilibrium is attained. The reaction at the point F takes
place between two solid phases and a solution saturated with
respect to both of them —

MNsT^ N + (solution of N in M)

THE DIAGRAM OF THERMAL EQUILIBRIUM 39

It is therefore likely to remain incomplete, owing to the com-
pound being deposited as an insoluble layer coating the crystals
of N and hindering further action. The methods of detecting
and allowing for such a condition of incomplete equilibrium
will be discussed later, in connection with the practical con-
struction of the thermal diagram, for the present it is sufficient
to say that the error is reduced to a minimum by very slow
cooling.

Atomic j>erceTita^e

Fig. 12.

cf N

In the diagram of the antimony-gold alloys (Fig. 13)^ it
will be seen that the break in the curve coincides with the
composition AuSba, and this is therefore one of the exceptional
cases referred to above. On cooling liquid alloys containing
less than 33-3 atomic per cent. Au, crystals of antimony separate,
but on reaching 460° a portion of these reacts with the mother-
Uquor to form crystals of the compound AuSba. From alloys
containing from 33'3 to 65 atomic per cent. Au, this compound
constitutes the primary crystallization, and from alloys richer
in gold, crystals of gold are the first to separate. Conversely,
on heating, the compound melts at 460°, decomposing at the
same time into antimony and a liquid alloy, so that F is an
invariant point, at which the equilibrium

AuSbalt Sb -f (liquid alloy of Au and Sb)

occurs.

' R. Vogel, ZHtsch. anorg. Chem., 1906, 50, 145.

4°

METALLOGRAPHY

More complicated conditions are very frequent. The
freezing-point curve may present several maxima, or several
breaks, or both maxima and breaks may occur on the same
curve. Examples of this are aiforded by the alloys of mag-
nesium and nickel,' and of potassium and mercury,^ represented
in Figs. 14 and 15 respectively. The former system has a

*Co 10 so 30 *0

' .1

J06f

100 XAXL-

hy weighb

Sb ^AwSb,

A u Sbi + A I

" ^a *0 60 SO 100

Atom,. % Aw
Fig. 13. — Antimony and gold.

well-marked maximum, corresponding with the compound
MgNia, and a break at 770°. Since the transformation at 770°
has a maximum duration at 33-3 atomic per cent. Ni, and the
eutectic arrest at 512° vanishes at the same concentration,
the existence of a compound Mg^Ni is clearly indicated. The
arrangement of phases is obvious from an inspection of the
figure.

' G. Voss, Zdtsch. anorg. Chem., 1908, 57, 34.

' E. Janecke, Zeitsch. physikal. Cheni.., 1907, 58, 245.

THE DIAGRAM OF THERMAL EQUILIBRIUM 41

•c<

■) 10 20

30 40 so

eo 70 ao

90 10

14.52°

ItOO

/

I30O

/

1100

Liquid'

W5°

/Nv
1 +

1100

\/

1000

Uguiab

^>^ waoj'^

900

aoo-

ssr

^ 770'

700 ■

1 \
1 \
/ \

MffzXl

MgzJfi

SOB

1 ZiowLcL
\1 SI2°

MgNii

soo

" — * .* ^

too

M0

y MgzNt

too /* ^i

by weight.

10 20 30 4-0 so eo TO GO SO 100

Fig. 14. — Magnesium and nickel.

■C ao ^o eo 70

'CO %fJff

by weight

Fig. 15. — Mercury and potassium.

42 METALLOGRAPHY

The system potassium-mercury (Fig. 15) is considerably
more complicated. There is only one maximum, that corre-
sponding with the remarkably stable compound HgaK, melting
216° higher than the less fusible of its two components. The
breaks in the curve, however, indicate no less than four other
compoundSj undergoing decomposition below their freezing-
points, namely, HgK, HgsK, HggKj, and HgjK. Time curves
were not recorded in this investigation.

Other examples of the kind of diagram described in this
section are given in the Appendix ; the formula of the inter-
metallic compound being giveri in each case.

CHAPTER III

THE DIAGRAM OF THERMAL EQUILIBRIUM {continued)
Solid Solutions or Mixed Crystals

In all the cases hitherto considered, the component metals or
their compounds have been assumed to crystallize from the
molten alloy in a pure state. More frequently, however, the
mutual solubility of the components which is evident in
the liquid state also persists, although usually to a smaller
extent, in the solid state. That is to say, in the majority of
cases the crystals of M which separate on cooling contain a
greater or less quantity of N, and this not mechanically retained,
but in a state of true solution. We may speak of true solution
in the case of solids whenever we find homogeneous crystals
of two or more components, the composition of which may be
varied continuously within certain limits. A " solid solution "
(the term is due to van't Hoff) is therefore a single phase.
Certain pairs of metals are isomorphous, that is, they crystallize
together in all proportions to forni homogeneous crystals, the
properties of which vary in a continuous manner from one end
of the series to the other. Others, although crystallizing in
different forms, are each capable of crystallizing with a small
quantity of the second metal, which is thus constrained to take
up a crystalline form foreign to it. The term " solid solution "
is sometimes retained for the cases in which there is no
resemblance of crystalline form, isomorphous mixtures being
placed in a separate class, but in the absence of any exact
knowledge of the distinction, it is better to group together
solid solutions of all kinds. Roozeboom introduced the word
" Mischkrystall " for a solid solution, both components of

43

44 METALLOGRAPHY

which are crystalHne, and this term is now very generally used.
The usual English rendering " mixed crystals " is in some ways
unfortunate, since it suggests to the mind rather a conglo-
merate than a single phase. For this reason, the older term
" solid solution " will be preferred in the present work.

It is probable that all metals should be regarded theoretically
as possessing some degree of mutual solubility, however slight,
in the solid state, the condition of complete insolubility con-
sidered in the preceding chapter being regarded as an ideal
limiting case. The quantity held in solution is, however,
frequently so small as to be negligible in practice. When
the solubility is too small to be recognized by thermal methods,
microscopical investigation affords valuable assistance.

The different types of equilibrium in which solid solutions
can co-exist with liquid alloys have been worked out by
Roozeboom^ and by Bruni.^ Whereas Bruni employed the
temperature^concentration diagram of which use has been
made in the preceding chapter, Roozeboom based his ex-
haustive study of the subject on the highly abstract principle
of the thermodynamical potential. Both authors arrived at
very similar results, and their conclusions may now be re-
garded as firmly established. For the practical purposes of
metallography, the method of the temperature-concentration
diagram is the only available one, and will be employed
exclusively, although the systems discussed will be referred to
the types of Roozeboom's classification, as being the more
complete and systematic.

The two component metals may be isomorphous, in which
case only a single series of solid solutions is formed, their
properties varying in a continuous manner from the one end
of the series to the other. It is easy to show that the freezing-
point curve of such a series must be continuous. The points
at which the direction of a freezing-point curve changes
suddenly in direction, so that two intersecting tangents to the
curve may be drawn at the same point, are invariant points,
at which the number of co-existing phases, vapour being

' H. W. Bakhuis Roozeboom, Zeitsch. physikal. Chcm., 1899, 30, 385.
' G. Bi^uni, Rend. R. Accad. Lincei, 1898, [v.] 7, ii. 138, 347.

SOLID SOLUTIONS

45

excluded, exceeds the number of components by one. A
series of solid solutions, however, constitutes but a single
phase, and since the only other phase present is the liquid
alloy, the number of co-existing phases never exceeds two.
By our simplified formula

/l = «— /+I = 2 — 24-1= I

that is, the system always has a^ degree of freedom, and the
freezing-point curve cannot exhibit a discontinuity.

Systems of this kind have been grouped in three different
classes, according as the freezing-point curve lies wholly

Fig. i6. — Solid solutions, Type I.

between the freezing-points of the two components (Type I.),
or presents either a maximum (Type II.) or a minimum (Type
III.). The first and third of these types are met with in
alloys.

Type I. — The freezing-points of alloys of isomorphous
metals frequently lie entirely between the freezing-points of
the pure metals. There is, then, no alloy in the series which
on freezing deposits crystals having the same composition as
the liquid with which they are in contact. The liquid phase
is always proportionately richer than the solid phase in that
component, the addition of which lowers the freezing-point.

In Fig. 1 6, the two isomorphous metals melting at A and

46 METALLOGRAPHY

B respectively, form a continuous series of solid solutions.
We will assume that the freezing-point curve is concave to the
concentration axis, as A/«B. An alloy containing M.x per cent,
of the metal N and Nx per cent, of the metal M will, on cool-
ing from the molten state, be represented by a point travelling
down Im. At the point m, crystallization sets in. But in
accordance with what has been said above, the crystals contain
comparatively less of the metal M than does the liquid, — they
have a composition represented, let us suppose, by n. As
freezing continues, the composition of the liquid is expressed
by a point travelling down mh. towards A, the solid phase
being successively represented by points on a second curve,
nph., which lies throughout its length to the right of AwB.
Roozeboom has called the curve which represents the com-
position of the liquid phase the " liquidus," and that which
represents the composition of the solid phase the " solidus."
The solidus lies entirely below the liquidus, and since the two
coincide at A and B, where both liquid and solid phases con-
sist of the pure metal, the solidus must always be less concave
or more convex towards the concentration axis than the
liquidus.

When the temperature has fallen to q, the last of the alloy
solidifies, and mq is called the " crystallization interval." But
it will be seen that the last portions of the liquid had the
composition f, and could not therefore deposit crystals of the
composition q. It follows that the solid phase is only repre-
sented by q as regards its average composition, unless some
further change occurs beyond those described above. The
further change, in conditions of complete equilibrium, is one of
diffusion in the solid phase. For example, the crystals
deposited when the composition of the liquid phase has
changed by an infinitesimal amount, say from m to m', will
have a composition represented by a point lying somewhat to
the left of ri. But the first crystals deposited had the com-
position n, so that the solid phase can only become homo-
geneous, a condition necessary for equilibrium, by diffusion,
the final result of which is the production of entirely homo-
geneous crystals of the composition ri. This readjustment by

SOLID SOLUTIONS 47

diffusion must take place at each stage in order that the solid
phase may remain homogeneous. Since diffusion in a solid
is very slow in comparison with crystallization from a liquid,
complete equilibrium is only attained when the cooling of the
alloy through the solidifying range of temperature is extremely
slow. This condition is rarely completely fulfilled in practice,
and the crystals deposited from the molten alloy therefore fall
short of complete uniformity of composition. It is possible,
by heating the alloy for a long time at a temperature some-
what below the melting-point, to cause diffusion to take place,
and such an "annealing" process is generally necessary to
destroy the heterogeneous structure of solid solutions.

If we suppose that no diffusion whatever takes place in the
solid state, but that crystals of different composition remain in
contact with one another without change, the solidified alloy
will be represented at successive temperatures by points lying
on a curve nr. It will still be true that the curve nq will
represent the average composition of the solid phase, but the
crystals will be composed of concentric layers of progressively
changing composition. In practice, the conditions are inter-
mediate between those of complete equilibrium and of entire
absence of diffusion, and the crystals obtained are therefore
heterogeneous, but less so than in the second hypothetical case.
The effect of such imperfect equilibrium on structure will be
discussed in connection with the microscopical study of alloys.

The process of melting reverses the course of events
observed on freezing. The alloy x remains solid until the
point q is reached on the solidus curve, when the first drop of
liquid appears, having the composition r if we assume con-
ditions of, complete equilibrium. The last portions of the
alloy liquefy at the point in. We see, therefore, that the
solidus curve, as well as representing the composition of
the solid phase in equilibrium with liquid at each temperature,
also indicates the temperature at which the first appearance of
liquid occurs when any alloy of the series is heated. It may
therefore be regarded as the melting-point curve, the liquidus
being the freezing-point curve.

Since the process of crystallization of a sohd solution is a

48

METALLOGRAPHY

gradual one, extending over an interval of temperature, the
cooling curve of such an alloy cannot have the same form as
that of an alloy belonging to a eutectiferous series, such as
those described in the last chapter. The characteristic form
is that shown in Fig. 17. The portion ah represents the cool-
ing of the liquid alloy. The first separation of crystals at b is
well marked, but the curve then assumes a rounded form, and
the final solidification of the remaining
mother-liquor, instead of producing a
horizontal arrest-line as in the case of
eutectiferous alloys, is only indicated
by a slight change of direction at c,
where the curve of solidification, be,
joins the cooling curve of the solid
alloy, cd. This change of direction is
sometimes difiScult to observe, the two
portions of the curve appearing to pass
smoothly into one another. An ap-
plication of Plato's method of cooling
(see p. 102) would probably increase
the distinctness of the break, but under
the ordinary conditions of working,
the determination of the solidus curve
and of the interval of solidification
are far less accurate than the other determinations needed in
constructiijg a thermal diagram.

Examples of complete isomorphism are most abundant
among the metals of high melting-point, such as the platinum
and iron groups, and gold, silver, and copper. The diagrams
of the alloys of gold and platinum (Fig. 18) and of copper and
nickel (Fig. 19) are typical of such isomorphous pairs of metals.
The interval between the liquidus and solidus curves may be
large or small. It will be noticed that the two curves are
much further apart in the gold-platinum series * than in the
copper-nickel series.'' A result of this is that rapidly cooled
alloys of copper and nickel are likely to be much more nearly

' F. Doerinckel, Zeitsch. anorg. Chem., 1907, 51, 333,
^ N. S. Kurnakoffand S. F. Schemtschuschny, ibid., 156.

A-xis of time
Fig. 17.

SOLID SOLUTIONS

49

homogeneous than alloys of gold and platinum cooled under
the same conditions, that is, the incompleteness of the equi-
librium has a smaller influence in the former case. If the
liquidus and solidus curves were to approach one another still
more closely, they might be imagined to coincide, in which
case the solid and liquid phase in equiUbrium with one another
at any temperature would be identical in composition. It is

•c

leoo

eao

TOO

Pt (I7f4'°)

lOO %Pt

by T*-eiffht

Au,fl064-°)

SoVidi soXvJtiori'
rAxu + Pt)

O 10 20 30 *0 SO eo 70 80 SO lOO

Atom-XPt
Fig. i8.— Gold and platimim.

very improbable that this case ever occurs in alloys, and there
are theoretical reasons for supposing it to be impossible.'

Type II., in which the freezing-point curve presents a
maximum, has only been observed so far in organic optical
isomerides, and not in alloys. The nature of the diagram
will be readily understood from a comparison with Type III.
The liquidus and solidus curves coincide at the maximum.

" J. J. van Laar, Zeitsch. physikal. Chem., 1906, 55, 435.

T.P.C.

5°

METALLOGRAPHY

Type III. — The metals form a continuous series of solid
solutions, the freezing-point curve passing through a minimum.

This type may be illustrated at once by a concrete example,
the alloys of copper and manganese (Fig. 20).^ The freezing-
point of each component is lowered by addition of the other,
but there is no eutectic point, the liquidus curve being con-
tinuous throughout. Where it passes through a minimum the

eoo

70O

eoo
soo

Ni (I4S£L

100 % Ni
by weight

Cu(l084-V

Solid/ soUuioTt.

(Cw + m)

20 30 *o SO

SO 100
Atom.'ANi

Fig. 19. — Coppet and nickel.

solidus and liquidus curves coincide, so that at that point the
solid and liquid phases have the same composition. At all
intermediate points on the curve the composition of the solid
phase differs from that of the liquid with which it is in contact,
the relation between the two being found by the method

' S. F. Schemtschuschny, G. Urazoff, and A. Rykowkoff, ZHtsch.
anorg. Chem., 1908, 57, 253.

SOLID SOLUTIONS

51

described on page 45. The remarks there made as to the
effect of imperfect equilibrium apply in this case also.

Since the alloy of minimum freezing-point solidifies at
constant temperature, its cooling curve has a horizontal portion
like that characteristic of a pure metal or a eutectic alloy. A
mere determination of the liquidus would in fact suggest that
the series was eutectiferous, the trough-like form of the curve
near the minimum not being readily distinguishable from the
intersection of two lines. A determination of the solidus

1300

60

1000

700

600

,ao zoo % Mn

hy Weight

Mn,(nBO°)

Solid, solution
(Cw + Mrv)

so 30 fo so 60 70 ao
Fig. 20. — Copper and manganese.

so 100

Atomic /•= Uln^

curve, however, enables the two conditions to be distinguished
with certainty. The eutectic horizontal is absent, the solidus
taking the form of a continuous curve, convex throughout to
the axis of concentration. As the accurate determination
of the solidus is a matter of considerable difficulty, this is
essentially a case in which the thermal and microscopical
investigation should go hand in hand, since the absence of the
characteristic eutectic structure, and the presence of homo-
geneous crystals of the solid solution at the minimum point,
should be readily detected by the latter method. The

52 METALLOGRAPHY

determination of the electrical conductivity is also a most
valuable aid in doubtful cases.'

How necessary such a control may be is well seen in the
case of the alloys of copper and gold. In spite of the impor-
tance of these alloys in coinage, their constitution has only
recently been studied in detail. The liquidus curve was
determined by W. Roberts-Austen and T. K. Rose,* who
regarded it as being composed of two branches, meeting in
a shallow eutectic trough. They were unable, however, to
observe any eutectic arrests on the cooling curves, or to deter-
mine any points on the solidus. A determination of both
liquidus and solidus' leads to the conclusion that the curve
is of the type just discussed, a continuous series of solid
solutions being formed. This is in accordance with the
evidence of the microscope* and of the electrical conductivity."

' S. Wologdine {Rev. de MHall., 1907, 4, 25) has given an entirely
different diagram for the copper-manganese alloys, in which a maximum
occurs, corresponding with the composition Cu^Mn, a eutectic being
formed on the manganese side of the maximum. This is, however, in-
consistent with the microscopical and electrical properties of the alloys.
Since Wologdine used charcoal to hinder oxidation, it seems probable
that his results are to be explained by the presence of carbide. This is
confirmed by the fact that his alloys, rich in manganese, disintegrated in
air, evolving hydrocarbons, a behaviour which is not observed with pure
alloys of the two metals. The data from which Fig. 20 was constructed
were obtained from alloys melted either in a current of hydrogen or under
a layer of barium chloride, the two methods giving identical results. The
results of R. Sahmen, Zeitsch. anorg. Chem., 1908, 57, i, obtained with
impure material, are in rough agreement with those of Schemtschuschny.

* Proc. Roy. Soc, 1900, 67, 105.

' N. S. Kurnakoff and S. F. Schemtschuschny, Zeitsch. anorg. Chem.,
1907, 64, 149.

* Roberts-Austen and Rose obtained photo-micrographs of certain of
these alloys in which a laminated structure, characteristic of eutectics, was
visible under very high magnifications. Nothing of the kind was observed
by the Russian investigators. There is thus a conflict of evidence, which
may be explained in one of three ways :

1. There may be a stable and a metastable system, only one of which

is eutectiferous, and either the one or the other may be obtained,
according to the conditions of cooling.

2. The system may be of Type V. (p. 55) with a very small gap

between the two solid solutions.

SOLID SOLUTIONS J3

The alloys of manganese and nickel form a series of
Type III., closely resembling those of copper and manganese.^

More complicated forms of the freezing-point curve, which
may present a point of inflexion, or both a maximum and a
minimum, may in theory be presented by alloys forming a
continuous series of solid solutions,' but such forms have not
been observed in metallic alloys in the absence of compounds ;
it is therefore unnecessary to discuss them here. All such
curves are subject to the condition that wherever the solidus
and liquidus coincide, so that the solid and liquid phases have
the same composition, the common tangent to the two curves
at that point must have a horizontal direction.

The next case to be considered is that in which the two
metals have orily a limited reciprocal solubility in the solid
state. If we compare a pair of isomorphous metals with two
completely miscible liquids, such as alcohol and water, the
alloys of the present type are comparable with a pair of liquids
such as water and ether, of which each can dissolve a certain
limited proportion of the other, so that all mixtures richer than
the limiting value separate into two layers, consisting of
saturated solutions of water in ether, and of ether in water
respectively. In the same way, certain pairs of metals may
form two series of solid solutions, and any alloys falling
between the limits of saturation must consist of a complex of
two phases, each of which is a saturated solid solution.

In the simplest case (Roozeboom's Type IV.) the two
series of solid solutions meet at a transition point, as indicated
in Fig. 21. The two branches of the freezing-point curve, AC
and CB, have each a corresponding solidus, AD and EB
respectively. At the temperature of the transition point C,
there are two solid phases in simultaneous equilibrium with the
liquid, C is therefore an invariant point, and the transition from

3. The supposed euteclic structure may be in reality a strain-structure
due to contraction during cooling, as observed in certain cases by
Beilby. The last explanation is the most probable,

» A. Matthiessen, Phil. Trans., 1861, 160, l6i.

' Schemtschuschny, Urazoff, and Rykowkoflf, loc. cit.

' R. Ruer, Zeitsch. physikal Chem., 1907, 69, I.

54

METALLOGRAPHY

one series of crystals to the other, indicated by the line CE,
must take place at constant temperature, that is, CE must be
horizontal. The arrangement of fields will be seen from the
diagram. All alloys lying to the left of C solidify as crystals
of the solid solution «, as in Type I. Similarly, alloys to the
right of E form only crystals of the solid solution /3. Alloys
between C and D at first deposit crystals of /3, but on cooling
past the transition temperature these are converted into the
stable form a. All alloys between D and E form, when solid,

M

Liguid

/ Liquid. /

c /

D /e
/

y^-if

/

j^liquiety

/

1 1 .

• a

' d + /3 1

A

l^i

Fig. 21.— Solid solutions, Type IV.

a complex of saturated a and /S crystals, having the compositions
D and E respectively. Such a complex is the exact analogue
of a mixture of water and ether which has separated into two
layers. Varying the composition of the alloy between the two
limits D and E changes only the relative proportion of the
two phases, without altering their concentration.

The lines DP and EQ have been drawn with a slight
incUnation from the vertical, since the solubility of one solid
metal in another, like that of liquids, usually decreases with
falling temperature, so that, sufficient time being given to

SOLID SOLUTIONS 55

establish equilibrium, the composition of the two saturated
solid solutions, a and /3, will change to a certain extent with
the temperature. The inclination of such lines has been
studied in a very few cases, the best known of which are the
alloys of copper with zinc and tin respectively. These cases
will be discussed fully later. The simple case, represented in
Fig. 2 1 has an excellent representative in the series cadmium-
mercury. The cadmium amalgams form two series of solid
solutions, and the ordinates of the points of intersection are —

A Pure Hg, -38°

C 65 atom, per cent. Cd, +190"

D 75

E 77 » ). n

B Pure Cd, +323°

The lines DP and EQ diverge at lower temperatures, the limits
of the two solid solutions being at 25°, for instance, 65 and 79
atom, per cent. Cd respectively. It was only found possible
to determine the liquidus curve by measurements with the
thermometer, the thermal changes indicated by the other hnes
of the diagram being so small as to escape observation, hence
it was found necessary to complete the investigation by studying
other physical properties of the amalgams. The position of
the solidus was in fact determined from measurements of volume
in the dilatometer, and that of the lines DP and EQ from
measurements of electromotive force.*

Liquidus and solidus curves of this type, although not often
found representing a complete series, such as that of the
cadmium amalgams, are of frequent occurrence when com-
pounds are present, in which case they naturally form only
a part of the entire equilibrium diagram.

The next case (Roozeboom's Type V.) occurs very fre-
quently. Two limited series of solid solutions are now
formed, between the limits of which a eutectiferous series of
alloys occurs (Fig. 22). The lettering corresponds with that

' H. Bijl, Zeitsch. fhysikal. Chenu, 1902, 41, 641. See also N. A.
Pushin, Zeitsch. anoig. Chem., 1901, 36, 201.

56

METALLOGRAPHY

of Fig. 21. The two branches of the liquidus now intersect at
a eutectic point, C, at which the two saturated solid solutions
are at once in equilibrium with the liquid phase. The eutectic
horizontal, DE, does not reach the limits of the diagram, as in
the cases considered in Chapter II., but stops short at the limits
of saturation of the two series of solid solutions. This is, in

B

Liquid y^ /

\P-

n y^ /

A

1

/^^ /

tx?^^^*^^

/ Uqutd /

^/ /

f D/

EL

1 a/

o + ^ 1 yS

M

x\ \p

N

Fig. 22.— Solid solutions, Type V.

fact, the means by which we detect the formation of solid
solutions in a eutectiferous series of alloys.

We will now consider what happens during the solidification
of alloys of this series. A liquid alloy represented by the
point p will, on cooling, deposit crystals of the solid solution
u., and its cooling curve will be of the form shown in Fig. 17.
A liquid alloy represented by /', however, at first deposits
crystals of a, and the composition of these crystals changes
from ^ to D as the temperature falls. At the point D, the a

SOLID SOLUTIONS

57

crystals are saturated, that is, they are incapable of taking up
any further quantity of the second metal. The point C is then
a eutectic point, at which the two phases D and E (saturated
o and ^ crystals respectively) separate simultaneously. The
cooling curve therefore differs from Fig. 1 7 in that the part be
is separated from cd by a horizontal portion corresponding
with the solidification of the eutectic. The curve of eutectic
arrest times is shown in the lower part of Fig. 22. When the
development of heat during the solidification of the eutectic is
small, extrapolation of the time curve to cut the zero line is

"Co

700
600

io 30 tc eo 60 70 60 90 100% Cu. ,

Solid, solution. A +
Solid eoUttiori'S

; B

iO ZO 30 *0

70 60 90 fOO

Atom,.yo Cu.

Fig. 23. — Silver and copper.

uncertain, and a more accurate determination of the position
of the points D and E is obtained by microscopical examination.
A good example of this type is furnished by the alloys of
silver and copper. The freezing-point curve was accurately
determined by Heycock and Neville,* who, however, did not fix
the position of the points D and E. The complete diagram,
constructed from the experimental data of Friedrich and
Leroux,^ is shown in Fig. 23. Since, however, the results of
the microscopical investigation of these alloys are not in agree-
ment with the thermal diagram, it would appear that equiUbrium
is by no means readily obtained. It is probable that the

> P&U. Trans., 1897, 189a, aj.
' Metallurgie, 1907, 4, 293.

58 METALLOGRAPHY

eutectic arrest was observed beyond its true limits, so that
the horizontal in Friedrich's diagram is too long. In draw-
ing Fig. 23, the electrical conductivity as determined by
Matthiessen^ has been taken into account in fixing the
probable limits of the formation of solid solutions. The
solidus in this figure must therefore be regarded as approxi-
mate only.

The changes which may take place in an alloy during cool-
ing are not at an end when the mass has solidified. Crystalli-
zation from solution, change from one solid phase to another of
different crystalline form, even chemical reactions between the
constituents^ are possible in the solid state without the presence
of a liquid solvent, although the velocity of such changes is in
general less than when a liquid is present. The first case to
be considered is that of a metal or inter-metallic compound
which exists in more than one crystalline form, having different
temperature ranges of stability. Such a substance is said to be
polymorphic ; when cooled, it shows a change of properties at
a definite temperature, the transition temperature, and the change
is generally accompanied by a development of heat, making
itself known as a further arrest in the cooling curve. Such
heat-changes are often comparatively slight, and are not readily
detected on the ordinary temperature-time curve, special
differential and other methods have therefore been devised for
their recognition, and will be described in the chapter on
practical thermal measurements.

The number of polymorphic metals and inter-metallic com-
pounds is very great, and the equilibrium diagrams of alloys
owe a large part of their complexity to this cause. When the
polymorphic metal or compound occurs in a eutectiferous
series, it undergoes the same change whether present as
primary crystals or as a constituent of the eutectic, and the
transformation takes place at the same temperature in both
cases. The polymorphic change is therefore represented by a

' Pogg. Ann., i860, 110, 190. The microscopical examination of
alloys of this series after annealing for different periods, by W. von
Lepkowski, Zeitsch. anorg. Chem., 1908, 59, 285, also points to the exist-
ence of solid solutions over a range similar to that indicated in the diagram.

SOLID SOLUTIONS 59

horizontal line extending over the same limits as the eutectic.
When, on the other handj the polymorphic metal forms solid
solutions, the transition temperature varies with the com-
position. Moreover, it has been shown by Roozeboom' that
the transformation of a solid solution, like its solidification,
must take place over a certain interval of temperature, so that
we have two curves, representing the beginning and the end of
the polymorphic change, and corresponding very closely with
the liquidus and solidus curves already studied. This will be
understood by reference to Figs. 24 and 25. In the former of
these, both the low and the high temperature modifications of
the two metals are assumed to be completely isomorphous. In
the second figure, only one of the components of the isomor-
phous series is assumed to undergo a change on cooling.

It may happen that the solid solution breaks up into its
components on cooling, and the diagram then assumes the
form shown in Fig. 26. The temperature at which each com-
ponent crystallizes from the solid solution is lowered by
addition of the other, and we consequently obtain two
transformation curves. The resemblance of this curve to the
freezing-point curve of a eutectiferous series is at once apparent,
and the analogy is a real one. Primary separation of the
components takes place along the two curves, and when the
temperature of intersection is reached, the remaining solid
solution splits up into a conglomerate of the two components.
From the resemblance of this alloy in its physical properties
as well as in its mode of formation to a eutectic, it is called
a eutedoid? The best-known examples are those of the iron-
carbon and copper-tin series, and these are illustrated in
Figs. 27 and 28. Fig. 27 is a small part of the complete
diagram of the iron-carbon alloys, constructed from the data
of Carpenter and Keeling.' The homogeneous solid solution

' Zeitsch. physikal. Chem., 1899, 30, 385.

' H. M. Howe, Metallographisi, 1903, 6, 249. The word "aeolic"
had been previously used, but not generally adopted. (See Howe, Iron,
Steel, and other Alloys, Boston, 1903.)

' H. C. H. Carpenter and B. F. E. Keeling, J. Iron Steel Inst.,
1904, i. 224.

6o METALLOGRAPHY

breaks up on cooling, setting free pure iron and iron carbide,
FesC. Pure iron separates along the left-hand curve, but at
760° a polymorphic change takes place, described as the trans-

djfntouadviBj^

9U'n<)7}U9dlU9X

f

"^

to

N «,

Ol

. 5*

1

gJ

^

a,

a

t>.

Ph

s

.<?

•IS

^

1\

Q

1

dl

,,'fl

<0

■Bwnqnouodxuej;

formation of /S- into a-iron. Since the metal is pure, this change
takes place at constant temperature. The remainder of the
curve now represents the separation of «- iron from the solid

SOLID SOLUTIONS

6i

solution. Passing to the alloys richer in carbon, the carbide
crystallizes along the right-hand branch, intersecting the other
at the eutectoid point, at which the remaining solid solution
splits up into a finely laminated conglomerate of iron and car-
bide, known as pearliU. All alloys comprised within the limits
of the diagram show an arrest on their cooling curves at 690°,
corresponding with the formation of pearlite. The heat-
development reaches a maximum at the eutectoid composition,
that is, at 0*89 per cent, of carbon.

eoo

soo

O OS I /■£ /-bVoC
Fig. 27. — Iron and carbon.

/s 20 Atom-

Fig. 28. — Copper and tin.

The curve in Fig. 28 is taken from Heycock and Neville's
diagram of the copper-tin alloys.* Alloys containing 17
atomic per cent, of tin contain a solid solution at 550°, which
on further cooling breaks up into crystals of copper and of
another substance, possibly Cu^Sn. The eutectoid composed
of these t<ro constituents has a very characteristic structure.
Both of these series of alloys will receive fuller treatment in a
subsequent chapter.

It was mentioned above that when two solid solutions are

' Phil. Trans., 1903, 803a, i. A slight modification has been intro-
duced, made necessary by the work of F. GioHtti and G. Tavanti, Gazzetta,
190S, 38, ii. 209.

62

METALLOGRAPHY

in equilibrium with one another, their composition usually
changes as the temperature falls, so that the lines separating
the different fields are not vertical, but inclined or curved.
This is well illustrated by the alloys of copper ^and zinc.
Fig. 29 is a portion of the diagram.' Alloys con taming 38 to
52 per cent, of zinc are homogeneous at 800°, forming the
/3 solid solution. At lower temperatures, those rich in copper
throw out crystals of a second solution, a, whilst those rich in
zinc throw out crystals of a compound, CuaZna. This crystalli-
C.

auu

\ ' /

700

v/

600

SOO-

Q(.+ /3

/3+y

400
300

4-0 t-s SO ss eo AtojTv.

%Zn/.
Fig. 29. — Copper and zinc.

zation proceeds as the temperature falls, so that the field of
the ^ solution becomes progressively narrower with decreasing
temperature.'

It is possible for the components of a solid solution to
enter into combination to form a definite compound on
cooling, or the components may each separately undergo a
polymorphic change, and the low-temperature modifications

> E. S. Shepherd, y. Physical Chem., 1904, 8, 421 ; V. E. Tafel,
Metallurgie, 1908, 5, 343.

' Later investigations (H. C. H. Carpenter and C. A. Edwards,
J. Inst. Maals,'igil, 5, 127 ; H. C. H. Carpenter, iiiii., 1912, 7, 70 ; 8, 51)
show that the condition represented above, although generally observed, is
not a stable one, and that a eutectoid point exists at 470°, below which
temperature the j3-solution has no stable existence.

SOLID SOLUTIONS

63

may combine. Such cases have been suspected, but not
accurately observed, in alloys of metals of high melting-
point. It will be well to examine a case of this kind,
selected from an investigation of mixtures of fused salts,^ in
order to indicate the kind of results that may be expected

900-

aoo

700

eoo

*oo

300 ■

K^SO^

sovyA

Solid solution^ A

rs j^cuz So 4, + 01 If 2 so^)

1^70°

A +J5

182

100 R?fa^So^+B

Solid, ioluiion
B

tHfd 'dtntble sa&J

O /O SO 30 40 so 60 70 60 90 tOO

Molec'AKi SO^
Fig. 30. — Sodium and potassium sulphates.

when such a u«action takes place. Mixtures of sodium
sulphate and ■g ilvcT sulphate solidify to a homogeneous series
of solid solutions, the freezing-point curve having a minimum
at 20 mol. per cent. K2SO4 (Fig. 30). Sodium sulphate
changes to a /3 modification at 234°, and potassium sulphate
at 595°. There are two eutectoid points, at 5 and 75 mol.
• R. Nacken, N. Jahrb. Min., Beil.-Bd., \<yyi, 24, i.

64 METALLOGRAPHY

per cent. KaSOj respectively, and between them the trans-
formation curve rises to a maximum corresponding with the
double salt NaaSO^.KjSO^,' which forms solid solutions to a
small extent with sodium, and to a much greater extent with
potassium, sulphate. The conditions of equilibrium are
readily seen on an inspection of the diagram. No parallel
case is known with certainty amongst alloys, but similar
conditions, namely the formation of a compound on cooling
a series of solid solutions, have been suspected in the course
of the study of alloys of cobalt and chromium.^

The occurrence of polymorphic change in the solid state
provides an additional means of fixing the formula of an inter-
metallic compound, since the alloy consisting of the pure
compound will show the maximum development of heat due
to its transformation. This fact often provides a means of
distinguishing between solid solutions containing a compound
and those which consist only of the component metals in an
uncombined state, Tammann's method of plotting arrest times
against composition being applied to the transformation. It
is unnecessary to give separate diagrams of such cases, as the
mode of application will be obvious from what has been said
above and will be further illustrated by concrete examples.

' Nacken (loc. cit.) does not regard this as a double salt, but only as a
solid solution having a maximum on the transformation curve, since he
finds that the density and crystalline form change continuously. The •
whole form of the diagram is, however, such as to indicate a compound.

' K. Lewkonja, Zeitsch. an org. Chem., 1907, 59, 293,

CHAPTER IV

THE DIAGRAM OF THERMAL EQUILIBRIUM
{continued)

Ternary and more Complex Alloys

The methods of investigating alloys of three or more metals
are the same in principle as when the simpler alloys of two
metals are dealt with, but the experimental difficulties, and the
complexity of the conditions which may present themselves,
are naturally much greater. For this reason, in spite of the
number of alloys of technical importance containing three
metals, very few ternary systems have been examined with
any degree of completeness, and the study of quaternary
alloys remains an untouched field. The thermal investigation
of any such complex system involves a very large number
of separate experiments, and all the sources of error met
with amongst binary alloys recur in an aggravated form, the
consequence being that the construction of the diagram of
thermal equilibrium of a ternary system is an undertaking of
considerable magnitude. The task is simplified when the
object of the research is technical rather than scientific, since
the alloys capable of being utilized in practice are most com-
monly confined within comparatively narrow Umits of composi-
tion, the remaining alloys of the system being useless for the
purposes of industry on account of brittleness or other undesir-
able properties. It is, therefore, probable that such investiga-
tions of limited regions, having a mainly practical object in
view, will assume greater importance in future metallographic
research, although it is to be hoped that investigators will also

T.P.C. ^5 F

66

ME TALLOGRAPHY

be found to carry out the complete study of a sufficient
number of these highly interesting systems.

The thermal equilibrium of a ternary system can only be
represented graphically by a diagram or model in three dimen-
sions. The method employed in metallography is that due in
the first place to Willard Gibbs/ but more generally associated

/

/

k

Y

A

/V

A

V

Y

\/\

'\

A

Aso

/}<j\

.

/\

y

X/

i0

\a\P

\

/

Y

\

/s^

^>^7\^^

/X

^^^J^

K

^

V

Y

y

A / X^>C° X"'

V

\

/\

/yo

Fig. 31. — Graphical representation of a ternary system.

with Stokes, who independently devised the same scheme,^
basing it on the method employed by Clerk Maxwell for the
composition of colours.^

The percentage compositions of the alloys are represented
by an equilateral triangle, the height of which is 100 (Fig. 31).
The three points A, B, and C there represent the pure metals.

' Trans. Connecticut Acad., 1876, 3, 176.

"^ Proc. Roy. Soc, 1891, 49, 174.

" Graphical methods of converting percentages by weight into atomic
percentages in ternary systems are given by F. Hoffmann, Metallurgie,
1912, 9, 133, and E. Jiinecke, ibid., 320,

TERNARY SYSTEMS 67

a point on the line AB represents a binary alloy of A and B,
and so on for the pairs AC and BC, whilst any point within
the triangle corresponds with an alloy of the three metals.
The composition of such an alloy is readily found by measur-
ing the perpendicular distance of the point from each of the
sides. Thus an alloy represented by X consists of 23 per
cent, of the metal A, 15 per cent, of B, and 62 per cent, of
C. The perpendiculars from A, B, and C on to the opposite
sides being each divided into 100 parts, the composition of
any alloy is expressed on the diagram by measuring distances
proportional to the percentages of two of the three constituents
present in directions parallel with the corresponding perpen-
diculars, the measurement of the third being superfluous, since
the sum of the three perpendiculars from a point are always
equal to the height of the triangle, that is to 100. It facilitates
plotting if the triangle is divided up into smaller triangles by
ruling lines parallel with the sides through each tenth gradua-
tion on the perpendiculars, as shown in the figure.

The ordinates of temperature are now erected as perpen-
diculars to the plane of the triangle, and their summits are
joined to form a surface or a system of surfaces, so that the
space-model ultimately obtained is a vertical triangular prism.
When only the freezing-point surface, or liquidus, is studied,
the representation in three dimensions may be dispensed with,
the diagram taking the form of a projection of the surface and
its contour lines on the basal plane, the temperatures being
written in at all important points. This graphical method will
be employed for the simpler cases. When, however, the solidus
surface and the temperatures of transformation in the solid state
have to be taken into account, such a projection becomes so
complicated as to be impracticable. The best general view
of such a system would be given by constructing the space-
model in a transparent material, such as celluloid, the various
surfaces being represented by plates of the material, coloured
if necessary, bent to the required shape and cemented in
place. Such a model would be best for purposes of demon-
stration, but a solid space-model is inconvenient when it is
required to read off the phase equilibrium of a given alloy,

68 METALLOGRAPHY

and some artifice becomes necessary in order that the results
may be expressed in the form of plane diagrams. We may
employ a series of projections on triangles representing the
basal plane, each projection corresponding with a transforma-
tion. A simpler plan, and one giving a much clearer view of
the equilibrium, is that of constructing a series of vertical
sections of the space-model. This has the further advantage
of coinciding with the plan usually adopted in the systematic
practical investigation of such alloys. For instance, a series
of alloys of A, B, and C are examined in which, while the
proportions of B and C are varied, that of A remains constant,
say lo per cent. The results obtained may be plotted to form
a plane thermal diagram, which is really a vertical section of
the space-model, cut through the point lo on A/, in a direc-
tion parallel with BC, so that its base is the line mn. A
second similar series is examined, containing 20 per cent. A,
and a second section, having the base ^r, is thus obtained.
A series of similar sections can be built up to give the space-
model. It is obvious that if, after the completion of the
experiments, it is required to bring out more clearly the
variation in the alloys brought about by the gradual increase
of B in the mixtures, the data already obtained allow plane
sections to be drawn parallel with AC. When reviewing such
a series of sectional diagrams, a little practice makes it easy
to grasp the spacial arrangement of the phases in the system.

The freezing-point surface alone is often rej)resented for
purposes of demonstration, by erecting the ordinates as thin
wires on a triangular wooden base, and filling up with plaster
of Paris, smoothing off at the level of the ends of the wires.
Perspective drawings or photographs of such plaster models
are often found in papers dealing with ternary systems.

The possible forms of equilibrium in ternary systems are
very numerous, and it is impossible to give an exhaustive
review of them. A list of the principal memoirs on the subject
is given below.' Only the two simplest cases will be discussed

' C. R. A, Wright and C. Thompson, Pnc. Roy. Soc, 1889, 45, 461 ;
1890, 48, 25 ; 1891, 49, 156 ; C. R. A. Wright, ibid., 50, 372 ; 1892, 62,
II, 530 ; 1894, 65, 130; W. D. Bancroft, J. Physical Chem., 1897, h

TERNARY SYSTEMS 69

here, and the application of the method to more complex cases
will then be illustrated by some concrete examples.

The two general cases to be considered are, firstly, that in
which the three metals crystallize from the molten alloy in the
pure state, neither solid solutions nor inter-metallic compounds
being formed ; and secondly, that in which the three metals
are isomorphous, forming an unbroken series of solid solutions
composed of any two or of all three of the components.

I. The Three Metals crystallize in a Pure State

If we assume that the only solid phases which separate on
cooling are the pure metals, neither compounds nor solid
solutions being formed, the three binary systems obtained by
taking the metals in pairs will each be of the form shown in
Fig. 4, p. 19, and the space-model will therefore be a triangular
prism, of which the three vertical faces are bounded at the
top by V-shaped curves. The freezing-point surfaces which
start from the angles of the prism intersect along three lines,
forming three valleys. The eutectic temperature of a binary
system may be regarded as the simultaneous freezing-point of
the two component metals, and the addition of a third metal
lowers this freezing-point, just as the addition of a second
substance lowers the freezing-point of a pure metal.

The curves of intersection, or the bottoms of the valleys,
therefore slope downwards from the outer faces to the interior
of the prism. They finally intersect at a point, the ternary eutectic
point, which necessarily represents a lower temperature than any
of the binary eutectics. It is on the existence of such a ternary
eutectic that the possibility of preparing the so-called " fusible

403 ; 1899, 3, 217 ; A. W. Browne, ibid., 1902, 6, 2S7 ; W. C. Geer, ibid.,
1904, 8, 257 ; E. S. Shepherd, ibid., 92 ; B. B. Kriloff, Zeitsck.
physikal. Chan., 1897, 24, 441 ; F. A. H. Scheinemakers, ibid., 1904,
60, 169 ; 1905, 51, S47 > 62, 513 ; H. W. B. Roozeboom and A.
H. W. Aten, ibid., 1905, S3, 449 ; G. Bruni, Gazzetta, 1898, 28, ii.
508 ; C. T. Haycock and F. H. Neville, Trans. Chem. Soc, 1891, 59,
936; L. Mascarelli, Atti R. Accad. Lined, 1907, [v.] 16, ii. 691 ; P.
Goerens, Metallurgie, 1909, 6, 531 ; N. Parravano and G. Sirovich,
Gazzetta, 191 1, 41, i, 417, 478, 569, 621 ; in addition to those mentioned
in this and the following chapter.

10 METALLOGRAPHY

metals," melting below roo", depends, a mixture of three
metals in the eutectic proportion in such a case melting more
readily than any possible mixture of the same metals taken
two at a time.

The well-known ternary system, lead — tin — bismuth, is
usually referred to this type.' Strictly speaking, the condition
that the solid phases separating shall be the pure metals is not
fulfilled here, since lead forms solid solutions to a limited
extent with tin,^ and a more thorough investigation would

Bismuth, tin, and lead.

probably show the same to be true of the other pairs, but this
fact does not affect the form of the freezing-point surface,
which alone falls to be considered here. A projection of the
surface on the basal plane of the prism gives the result shown
in Fig. 32, in which the positions of the binary eutectic lines
and of the ternary eutectic point are clearly seen.' The dotted
curves are isothermals, drawn at equal intervals of temperature,

' G. Cliarpy, Compt. rend., 1898, 126, 1569 ; Bull. Soc. cV Encourage'
vunt, 1898, [v.] 3, 670.

^ W. Rosenhain and P. A. Tuclcer, Phil. Trans., 1908, 209a, 89.

' Tlie diagram is talcen from Cliarpy, corrected by the later results of
E. S. Shepherd, J. Physical Chem., 1902, 6, 519, and recalculated into
atomic percentages.

TERNARY SYSTEMS 71

•

and connecting alloys of equal initial freezing-point ; they may
be best realized by considering them as the contour-lines of the
solid model. As in a map, these contour-lines indicate the
slope of the surface, which is steep when the lines are closely
crowded together, and gradual when they are widely separated.

The process of cryztallization of an alloy belonging to this
system may now be followed in detail. If we consider a
liquid alloy, the composition of which is represented by the
point A, we shall see that the cooling of such an alloy will be
represented by a point travelling down the perpendicular
erected on the triangular base at the point A. This perpen-
dicular cuts the freezing-point surface BiCFD, that is, the
surface corresponding with the crystallization of bismuth. The
position of the isothermal indicates that the separation of
bismuth begins at 200°. The passage of a part of the bismuth
from the liquid to the solid state does not alter the relative
proportions of lead and tin in the portion remaining liquid,
and the change of composition of the liquid alloy with falling
temperature is therefore represented by a point travelling
along the line BiAG from A towards G. When G is reached,
this line intersects the eutectic line CF, which represents
a system in which solid bismuth and solid tin are simultane-
ously in equilibrium with the liquid mother-liquor. A eutectic
alloy of bismuth and tin therefore separates. The relative
proportion of the two solid phases in the eutectic changes
somewhat as the temperature falls, as is indicated by the line
CF, which is not straight, but slightly curved. When the
point F is reached, the whole of the remaining alloy solidifies
at constant temperature. The ternary point F corresponds
with the simultaneous equilibrium of one liquid and three
solid phases, namely, the three pure component metals.

Examination shows that the process described accords
with the indications of the phase doctrine. We are now
dealing with a system of three components. When solid
bismuth separates, there are two phases present, namely, the
crystals of the solid metal, and the still liquid alloy. Since

/' = «-/+! (P- 25)

72 METALLOGRAPHY

f = 2, or the system is divariaiit, and temperature and com-
position may be varied independently. When the first eutectic
line is cut, a new phase appears, namely, soUd tin. The
number of phases being now three, the system is only uni-
variant, so that, the temperature being given, the composition
is also determined. At the ternary eutectic point F, four
phases are in equilibrium, namely, the three solid metals and
the liquid ternary eutectic, so that

/' = 3 - 4 + I = o
the system is invariant, and can exist only at one definite
temperature, the temperature indicated by F. It wilt be
noticed that, on the space-model, bivariant systems are repre-
sented by surfaces, univariant systems by lines,' and invariant
systems, of which only one is possible in the case considered,
by points. A three-dimensional region, such as that existing
above the freezing-point surface, represents a tervariant system,
since the concentration of two components, and the tempe-
rature, may be varied independently. Below the ternary
eutectic temperature, three solid phases co-exist, and since we
have assumed them to be perfectly immiscible, no change of
composition can be brought about, and the phase rule ceases
to be applicable. It again finds application in the cases to
be considered immediately, in which the crystalline phases are
not pure metals, but solid solutions, the composition of which
depends on the temperature. To bring all such cases under
the phase rule, we must assume that the solid metals are not
completely immiscible, but that solid solutions are formed, if
only to an inappreciably small extent. We then have three
co-existing phases, and the system is univariant, so that a
change of temperature brings about a change in the con-
centration of the solid solutions. The miscibility of solid
components is a thermodynamical necessity, although it may
occur only to an infinitesimal extent.^

II. The Three Metals are Isomorphous
A continuous series of solid solutions is formed, and the
freezing-point surface is a continuous one, bounded by the three
' R. Ruer, Zeitsch, physikal. Chem., 1908, 64, 357.

TERNARY SYSTEMS 73

freezing-point curves of the three binary systems. The pro-
jection on the basal plane shows no eutectic lines or points.
The curvature of the surface may be indicated by means of
contour lines. No experimental study of such a system yet
exists, but there can be no doubt that the alloys of closely
related isomorphous metals, taken in threes, will be found to
conform to this type. Such, for instance, are the metals of
the platinum series, togetljer with gold. Thus, alloys of
platinum, palladium, and gold, may be expected to form a
ternary system of this simple type.

III. The Alloys contain Solid Solutions

A good example of a ternary series in which solid solutions
are formed to a limited extent, no compounds being formed,
is furnished by the mercury amalgams of cadmium and lead.*
The three binary systems are of a simple type, and it is not
necessary to reproduce the corresponding diagrams separately.
Cadmium and lead form a eutectiferous series, the eutectic
point lying at 249° and 33 atom, per cent. Cd. Each of these
metals is capable of retaining small quantities of the other in
solid solution, lead holding up to 4 atom, per cent, cadmium,
and cadmium holding a very small percentage of lead. In the
lead-mercury series, the eutectic point lies so near to the
mercury end of the curve as to be indistinguishable from it,
and solid solutions are also formed up to a concentration of
about 40 atom, per cent. Hg. The cadmium-mercury diagram
has been described in a previous chapter'' Fig. 33 shows,
above, an elevation of the space-model, and, below, the
projection of the same on the base. The latter is divided into
six fields, each of which corresponds with the primary separa-
tion of a distinct crystalline constituent from the molten alloy.
The Hues separating these fields, and the points at which these
lines intersect, indicate the temperatures and concentrations
at which different phases are in equilibrium. There is no
ternary eutectic point, and only a single binary eutectic curve,

' E. Janecke, Zdtsch. physikal Chtm., 1907, 60, 399 ; 1910, 73, 328.
" Page li-

74

METALLOGRAPHY

(tA, namely, that separating the regions of crystaUization of
lead and cadmium. The points k and D are not to be
distinguished from the freezing-point of mercury, the second
solution in the lead-mercury system being practically pure
mercury, but for the sake of completeness their distance from
the angle Hg of the triangle is exaggerated. The meaning of
the points of intersection of the eutectic and transformation
curves is shown in the following table : —

Point.

Character.

Phases in equilibrum with

liquid.

Temp. "C.

a
b
k
A
B
C
D

Eutectic
Transformation

Pb, Cd

CdHgo, HgCd/3

PbHgo, HgPb/3

Pb, Cd, CdHgo

Pb, Cd, HgCd3

Pb, PbHgo, HgCd;3

PbHgo, HgPb^, HgCd;3

249

188

-40

23s
169

50
-40

The general behaviour of the alloys during solidification
will be seen from a comparison of the upper and lower
diagrams in Fig. 33. Consider an alloy represented by a
point within the area aACd. As the temperature falls, it will
deposit crystals of cadmium. Its composition then changes in
the manner described in connection with the alloys of lead,
tin, and bismuth (p. 71); .the point representing it travels
down the freezing-point surface until it reaches the line aA.
The binary eutectic of lead and cadmium then separates, and
the temperature continues to fall until the point A is reached.
This is not a ternary eutectic point, but represents a reaction
between the crystals and the mother-liquor, in accordance with
the equation

Cd + a solution of composition A = Pb 4- CdHga

The temperature remains constant at 235" so long as this
reaction continues. If the cadmium is in excess, the mother-
liquor solidifies completely at this temperature; but if the
cadmium is used up in the reaction, and some liquid of com-
position A remains, this will fall in temperature, depositing

TERNARY SYSTEMS 7S

Cd,

Fig. 33. — Lead, cadmium, and mercury.

76 METALLOGRAPHY

simultaneously crystals of Pb and CdHga along the line AB.
The other lines on the diagram are explained in the same way.

Tammann's method of thermal analysis may also be applied
to ternary systems.^ The experimental difficulties are naturally
greater than when dealing with binary systems, and the fact
that several arrests may occur on the cooling curve of a single
alloy necessitates the use of delicate methods of observation
and of a carefully considered scheme for the plotting of the
data obtained. It is desirable to make up series of alloys on
a systematic plan, choosing their compositions so that each
series constitutes a plane vertical section through the space-
model ; the arrest-times are then plotted for each plane section
exactly as for a binary system. Two plans have been adopted
by different workers for the arrangement of these sections.
Each series may be arranged to contain a constant proportion
of one component metal, say A, whilst the relative quantities
of B and C are progressively varied. This is equivalent to
constructing sections parallel with one of the sides of the
triangle of projection. Or, on the other hand, the ratio of B
to C in each series may be kept constant, a progressive increase
being made in the proportion of A. This is equivalent to
constructing sections starting from one angle of the triangle,
and diverging. Each method has its advantages. The first
is, perhaps, to be preferred, as being the simpler in practice;
and if a sufficient number of alloys are taken, it is not difficult,
from the data obtained by the first method, to construct plane
diagrams in accordance with the second plan, when such are
required to emphasize a particular fact, such as the influence
of a binary compound on the equilibrium.

As an example of a system which has been studied in this
way, we may take the alloys of magnesium,' lead, and tin.^
One of the three binary systems has been already described
(P- 35)- The systems Mg— -Pb and Mg— Sn resemble one

' R. Sahmen and A. von Vegesack, Zeitsch. physikal. C/iem., 1907, 59,
257 ; 1908, 60, 507 ; E. Janecke, ibid., 1907, 89, 697. The controversy
between these authors deals merely with the possibility or impossibility of
the occurrence of certain types of ternary systems amongst alloys.

^ A. von Vegesack, Zeitsch. anorg. Chem., 1907, S4, 367.

TERNARY SYSTEMS 77

another very closely, each containing a binary compound
of the same type, solid solutions not being formed to an
appreciable extent. As regards the system Pb — Sn, Vegesack,
working with somewhat rapid cooling, did not detect the
formation of the solid solution which undoubtedly exists ; but
this error does not noticeably alter the form of the liquidus
surface.

The results are shown in Fig. 34, the upper diagram in
which represents in elevation the space-model, and the lower
the projection of the liquidus on the basal plane. The two
compounds, MgaSn and MgaPb, form solid solutions with one
another, the solutions forming J wo series with a small gap, that
is, belonging to the Type IV. (p. 53). A vertical section
along the line db therefore represents a binary system of pre-
cisely the same type as the cadmium amalgams (p. 55). The
width of the gap between the two solid solutions is, however,
increased by the addition of lead or tin to the alloy, so that,
near the eutectic line hfe the a crystals may be regarded
without serious error as pure MggSn, and the ^ crystals as
pure MgaPb. A section on the line aPb will then show a
freezing-point curve corresponding with a simple eutectiferous
series, the two components of which are MgaSn and Pb. The
arrest corresponding with the eutectic solidification will occur
at constant temperature, and will have a maximum duration at
the composition / Fig. 35 represents the sections on ab and
rtPb respectively, with the arrest time- concentration curves
dotted. In the second diagram the atomic percentages of lead
only are indicated on the line of abscissae. Since the starting-
point on the left-hand side is the point a, representing the
compound MgaSn, the tin and magnesium are in the constant
atomic ratio i : 2 throughout, and may be calculated with
ease.^

At the ternary eutectic point h (Fig. 34), three solid
phases, Sn, Pb, and a crystals (practically pure MgaSn), are in

' In the second diagram, the dotted curve is somewhat idealized.
Owing to the rapid rate of cooling, Vegesack did not obtain complete
equilibrium, and his eutectic line vanished before reaching the limit of the
diagram.

78

METALLOGRAPHY

Fig. 34.— Tin, magnesium, and lead.

TERNARY SYSTEMS

79

equilibrium with the liquid. There should be two other invariant
points in the ternary system, corresponding with the equilibrium
of Mg, a, and ^, and with that of a, p, and Pb respectively.
These points are, however, found to coincide with the binary
eutectic points d and e respectively, thus introducing a certain
simplification into the diagram. Only one ternary eutectic
mixture is therefore formed.

It will be sufficient to examine the process of solidification

Mg§n

lOO

Mol°/aMg^Pb.

Fig. 35. — Sections through the model (Fig. 34).

100
Atom'/oPb

in two cases. Consider first a point lying somewhat to the left
of the line aPb in the triangular projection (Fig. 34). An alloy
of this composition will, on cooling, deposit crystals of the
a solution, which will continually adjust their composition to
maintain equilibrium with the liquid, until, when the line /if is
approached, they consist, as mentioned above, of practically
pure MgaSn. When a point on hf is reached, crystals of lead
begin to separate, a binary eutectic of lead and MgjSn being
thus formed. The temperature continues to fall, as indicated
by the arrow-head, until the point h is reached, when the
whole of the mother-liquor solidifies at constant temperature as
the ternary eutectic.

8o METALLOGRAPHY

If, on the other hand, the point representing the compo-
sition of the alloy lies between the lines ab and «Pb, the first
crystals deposited are, as before, the a solution ; but the line
dee, when intersected, is not a eutectic line, but one of trans-
formation. When it is reached, the a crystals react with the
mother-liquor, according to the equation

a + liquid 3_;8

VVith the progressive formation of ;8 crystals, the temperature
falls towards e, at which a third solid phase appears, namely
lead, and the remaining liquid solidifies at constant tempera-
ture. The behaviour of any other alloy of the series may be
predicted by similar reasoning.

Quaternary systems are represented graphically by means
of a regular tetrahedron, of which the four component ternary
systems occupy the faces. The methods of constructing such
a figure, and of representing the results of thermal analysis in
space or on a series of plane sections are discussed by
N. Parravano and G. Sirovich,^ and J. M. Bell.^

' Afti. R. Accad. Lincei, 191 1 [v] SO, ii, 206, 331, 412.
'^ J. Physical Chein., 1911, 15, 580.

CHAPTER V

THERMAL EQUILIBRIUM OF METALS WHICH ARE ONLY
PARTIALLY MISCIBLE IN THE LIQUID STATE

All the alloys hitherto considered have been assumed to melt
to a homogeneous liquid. This is, however, by no means a
universal condition. Whilst many pairs of liquid metals
resemble alcohol and water, mixing in all proportions to form
a single liquid phase, others behave like water and ether, the
mixtures, within certain limits of concentration and temperature,
separating into two immiscible liquid phases. All degrees of
miscibility, from the practically complete mutual insolubility of
water and mercury to complete miscibility, are observed.
Amongst metals, certain pairs, such as lead and aluminium,
behave, at temperatures only slightly above their melting-
points, like mercury and water, separating into two layers,
each of which consists of one of the metals in a practically
pure state. A more common case is that in which each metal
shows a certain limited power of dissolving the other, so that
the two liquid layers consist of dilute solution of A in B, and
of B in A respectively. Complete immiscibility cannot be said
strictly to occur, the apparent instances of such a condition
being regarded as cases in which the reciprocal solubility of the
two components is very small.

This last statement will be understood after a consideration
of the effect of temperature on the equilibrium of partially
miscible liquid phases. Whilst the reciprocal solubility of two
liquids may either increase or decrease with rise of ternpera-
ture," only the former case has been observed in alloys, and it

' See A. Findlay, The Phase Rule, 99.
T.P.C. 8'

82 METALLOGRAPHY

is, according to our knowledge of the general properties of
metals, exceedingly improbable that the reverse condition
should ever present itself. As the temperature rises, then,
the liquid layer in which A is in excess becomes increas-
ingly richer in B, and conversely, the layer in which B
is in excess becomes increasingly richer in A, that is, the
compositions of the two layers tend to become more nearly
equal. This points to the existence of an upper limit of
temperature, at which the two liquid phases would become
identical in composition, resulting in the formation of a single
homogeneous liquid. Such a critical solution temperature
probably exists in all cases of partial miscibility, although its
experimental realization is a matter of extreme difficulty, owing
partly to the fact that many of the metals forming alloys of this
type are volatile, so that the boiling-point is reached below the
critical temperature, and partly to the difficulty of observing
the formation of layers directly, owing to the absence of trans-
parency. For these reasons, the critical phenomena in liquid
alloys have been very little studied, although the increase of
reciprocal solubility with rising temperature has been noticed
in several cases. The few measurements which exist' are
more or less inaccurate, owing to imperfect separation of the
two layers. When the alloys are rapidly cooled and analysed
after solidification, time is often afforded for a change in com-
position to take place, whilst attempts to separate the still liquid
alloys by means of a pipette often fail through the tendency of
the heavier alloy to remain suspended in globules in the lighter
layer.

If we consider two metals, the miscibility of which at a
temperature slightly above the melting-point of the less fusible
metal is so small as to escape observation, the process of
freezing of any mixture of the two will be a very simple one.
The freezing-point of each metal will be entirely unaffected by
the presence of the other, and will thus be the same, whatever
be the composition of the mixture taken. The whole of the

• W. Spring and L. Romanoff, Zeitsch. anorg. Chem., 1896, 13, 29
(lead-zinc and zinc-bismuth, critical temperatures determined) ; C. R.
Alder Wright, Proc. Roy. Soc, 1891, 49, 156 (bismuth-zinc).

PARTIALLY MISCIBLE LIQUIDS

83

less fusible metal thus crystallizes at its normal freezing-point,
after which the temperature falls to the freezing-point of the
second metal. This may be represented in a diagram as in
Fig, 36, and does not call for further remark.

If the two metals have a distinctly measurable miscibility in
the molten state, the diagram will, in the simplest case, take
the form shown in Fig. 37. The freezing-point of A will be
depressed by the addition of small quantities of B \ and con-

Li

quA.d>

B +

liquieL

A

+ B

Fig. 36.— Completely immiscible liquids.

versely, that of B will be depressed by the addition of A. The
eutectic temperature will probably lie near to the freezing-point
of the more fusible metal, as at C ; and in fact, in the observed
cases of alloys belonging to this type, the eutectic point usually
lies so near to the end of the series as to be practically indis-
tinguishable from the freezing-point of the pure metal. Thus,
in the alloys of copper and lead (Fig. 38 '), the freezing-point
of lead is only lowered to the extent of i"i7", the eutectic
mixture containing only o*o6 per cent, of copper.^

' C. T. Heycock and F. H. Neville, Phil.Trans,, 1897, 189a, 25.
» Trans. Chem. Soc, 1892, 61, 888,

84

METALLOGRAPHY

From Fig. 37, in which the assumption is made that both
metals crystallize in a pure state, neither compounds nor solid
solutions being formed, it will be seen that liquid alloys rich
in B will first deposit crystals of B, the temperature falling as
indicated by the curve BE. This continues until the liquid
reaches the temperature and concentration represented by the
point E, at which the freezing-point curve intersects the misci-
bility curve, shown as a dotted line. At this point the mother-
liquor, if it continued to deposit crystals of B, would attain a

B

+ tiquia/3

/
di

1

Liquid, a

\
e. \ liquid.

1 /

Ligtud-
/

1
1

+ liquid 13

•/

/d-

-~-,^_

1

/

B + Hguid a

■--.^

A

. /

tiguid

V

c

A ■¥ B

Ficj. 37. — Partial miscibility.

composition represented by a point lying somewhere between
D and E. The diagram shows, however, that all such points
lie within the region of immiscibility, and the result of the
crystallization must therefore be the formation of a second
liquid phase having the composition of the alloy D. There
are thus three phases in contact with one another, namely,
the solid metal B and the two liquid phases D and E. The
system is consequently invariant, and the further crystallization
must take place at constant temperature. This is the meaning
of the horizontal line DE. As crystaUization proceeds, the

PARTIALLY MISCIBLE LIQUIDS

85

mean composition of the part of alloy remaining liquid is
represented by a point travelling along this line from E to
D, until D is reached, and the liquid alloy E disappears.
There are now only two phases present, and any further with-
drawal of heat has the effect of lowering the temperature, the
metal B now crystallizing from a homogeneous liquid along
the branch DC until the eutectic point C is reached, after which

tveigliC

lOa*'

1100

10

ZO 30 40

so 60 TOBBSan

' 1

1000

Liqu^'41.

1
1

Two liquids

900

^

95r°

BOO

/

700

600

/

Cu + liquid.

500

/

'

400

'Pb

3Z6°

300

200

Pb+ Cu

100

20 30 40 50 60 70 80

Fig. 38. — Copper and lead.

00 100^,
Atom, /o
Cu.

the process continues in a perfectly normal fashion. On the
individual cooling-curves of alloys of this system, the formation
of the second liquid phase is marked by a horizontal portior,
exactly resembling the arrest due to the solidification of a
eutectic. Microscopical examination, however, generally
sufifices to distinguish the two cases. The maximum duration
of this arrest will be shown by the alloy having exactly the
composition of E, and the duration will become zero for the
alloy having the composition D, as indicated by the curve D/E.

86 METALLOGRAPHY

If the liquid alloy has originally a composition between D
and E, the separation into two layers takes place before the
crystallization of solid metal begins. As crystals of B separate
from the liquid alloy E, a larger quantity of the conjugate alloy
D is formed, and this process continues until the whole of E
has disappeared. The curve of miscibility, D^ijE, has not been
represented as closed, as the maximum, the temperature, that
is to say, at which the composition of the two liquid phases
becomes identical, is in general so high as to be unrealizable
under the conditions of the experiment, and to represent the
branches Y)d and Ee as meeting at a critical point within the
limits of the diagram would therefore be misleading. In most
of the alloys of this type hitherto observed, also, the branch DC
is very steep, so that the amount of the metal B crystallizing
during the fall of temperature from that of the horizontal DE
to that of the eutectic point is usually very small, and the
arrest on the cooling curve is correspondingly difficult to
measure.

It is not necessary to enter minutely into the nature of the
modifications which it is necessary to make in such a. diagram
in order to embrace the cases in which either solid solutions
or inter-metallic compounds are formed. A comparison of the
diagrams already discussed will render the construction of such
a complex diagram for any particular case an easy matter.
The form of the equilibrium diagrams and arrest curves in
such cases has been treated of by Tammann.^ A single
example, the sodium-cadmium series,^ is represented in Fig. 39.
Two compounds are formed, giving rise to maxima in the
freezing-point curve, and there is a small region of partial
miscibility between the compound Cd2Na and sodium. The
formula of the second compound is doubtful ; it may prove to
be CdgNa instead of CdjNa, as represented in the diagram.

' Zeitsch. anorg. Chan., 1905, 47, 289.

"^ C. H. Mathewson, ibid., igo6, 60, 171 ; N. S. Kurnakoffand A. N.
Kusnetzoff, ibid., 1907, S2, 17J,

PARTIALLY MISCIBLE LIQUIDS

87

Ternary Systems in which a Region of Partial
miscibility occurs

The formation of immiscible liquid phases assumes its
chief practical importance in systems composed of three
metals. For the study of such systems, we make use of

♦00

300

10

so

aO 4« so 60 BO leO^

100

so so 100

Atom'/o

Na

Fig. 39. — Cadmium and sodium,

the triangular diagram already explained and employed in
Chapter IV. to express the results of the thermal analysis of
ternary systems.

When, to a mixture of two metal's, such as molten lead
and zinc, a small quantity of a third metal is added, the
mutual solubility of the two original metals is changed. For
instance, if the added metal is tin, the solubility of lead in
zinc, and of zinc in lead, will be increased by the addition,
and at the same time both liquid layers will be found to

88 METALLOGRAPHY

contain tin. The result is that the two layers approach more
nearly to equality of composition as the quantity of tin is
increased, until finally a point is reached at which the two
liquid alloys become identical, and a single homogeneous
liquid phase is formed. If we now construct a triangular diagram
showing the composition of the liquid phases at some given
temperature, we obtain a result such as that shown in Fig. 40,'
which represents the results obtained with mixtures of lead,
zinc, and tin at 650°. A mixture, the original composition of
which is expressed by a point falling within the curved

H K "

Fig. 40. — Isothermal of tin, lead, and zinc at 650°.

boundary, as at P, separates into two liquid layers, the
composition of which at 650° is represented by Q and R
respectively. The alloys Q and R are then conjugate alloys.
The line joining them must pass through P, and was called
by Stokes a tie-line or tie. As the proportion of tin is increased,
the ties become shorter, finally vanishing at the point L. At
this point the two liquid phases become identical in composi-
tion, and the mixture becomes homogeneous. We thus see
that all points in the triangle lying outside the curve HQERK
represent homogeneous liquid alloys ; whilst points within this
curve represent alloys which, at the given temperature, can

' C. R. Alder Wright, C. Thompson, and J. T. Leon, Proc. Roy. Soc,
1891, 49, 174.

PARTIALLY MISCIBLE LIQUIDS 89

only exist in the form of two co-existent liquid phases. The
curve separating the two regions is the critical curve, and points
falling within it represent what are termed by Alder Wright ideal
alloys.

The diagram in Fig. 40 is an isothermal diagram, that is,
it represents the state of things at a certain constant tempera-
ture, in this case 650°. If we construct similar triangular
diagrams for a number of different temperatures, we shall find
that the area enclosed by the critical curve diminishes with
increase of temperature, in accordance with the increase of
mutual solubility of two liquid phases when heated. At a
sufficiently high temperature it would vanish entirely, and all
mixtures of the three metals, whatever their relative pro-
portions, would form a homogeneous liquid. In the space-
model of the ternary system, the critical surface will thus be
closed above, and the highest point of the surface will
evidently be the critical point of the binary system which
yields immiscible liquid phases, in this case lead-zinc. So
far, no such case has been experimentally studied among
alloys.^

In accordance with the doctrine of phases, the composition
of two conjugate alloys should be independent of their relative
quantity. Consequently, if we take equal weights of mixtures
containing lead and zinc in the proportions 2:1, 1:1,
and I : 2 respectively, and add the same quantity of tin to
each mixture, we should obtain liquid layers having the same
composition in each case, and only differing in their relative
quantity. This was shown by Stokes, in the paper referred to,
to be a necessary consequence of the principle of equilibrium
involved. The results of Wright, Thompson, and Leon show
that this result is not fully realized in practice. The reason
lies in the experimental difficulties of the investigation. Very
thorough stirring is necessary in order that the two liquid
layers may come into contact sufficiently to ensure equilibrium,
especially when, as in the case of lead and zinc, the two liquid

' For the graphical representation of such a system, with examples
from organic mixtures, see P. A. Meerburg, Zeitsch. physikctl, Chem., 1902,
40, 641.

90 METALLOGRAPHY

metals differ widely in density. Further, when equilibrium has
once been attained at a definite temperature, and sufficient
time has been allowed to elapse for complete separation of the
two layers to take place, the withdrawal of samples of those
layers for analysis presents some difficulty, since, as the alloy
cools, a readjustment of the equilibrium takes place. In the
investigations mentioned above, the alloys were cooled rapidly,
samples being taken from the solidified upper and lower layers
after complete cooling, it being assumed that no marked
change of composition had taken place in the short period of
cooling. It is probable that this assumption is not justified,'
and in future investigations it is desirable that samples should
be taken at the temperature of the experiment by means of
suitable pipettes.^ A method which is perhaps simpler in its
application consists in the employment of a special furnace,
which allows samples of the contents to be withdrawn by
tapping at different levels.'

An equilibrium of this kind may be looked at from another
point of view. We may require to consider, instead of the
influence of a third metal in altering the mutual solubilities of
two others, the proportion in which the third metal is dis-
tributed between the two others; In the case already men-
tioned, tin dissolves in both the lead layer and the zinc layer.
The ratio of the concentrations of tin in the upper and the
lower layer may be called the partition-coefficient of tin between
lead and zinc. The concentration of tin in the two liquid
phases is not usually the same, and the tie-lines are therefore
not, in general, parallel to the base of the triangle ; the critical
curve is consequently more or less asymmetrical. The experi-
mental data at present available are very scanty, and are quite
insufficient to allow us to calculate the partition- coefficient with
any approach to accuracy. The coefficient will be affected by
the formation of compounds between the added metal and
either of the original, partially miscible metals.

An interesting case of this kind, which has been studied in

' W. D. Bancroft, J. Physical Chem., 1899, 3, 217.
« G. N. Potdar, J. Coll. Sci. Tokyo, 1908, 25, ix.
' K. Friedrich, Metallurgie, 1906, 3, 396.

PARTIALLY MISCIBLE LIQUIDS 91

detail on account of its technical importance, is presented by
the alloys -of lead, zinc, and silver. Silver mixes in all pro-
portions in the liquid state with either lead or zinc, forming
compounds, however, with the latter,^ whilst yielding only a
simple eutectiferous series with the former (p. 31). In the
ternary system, the distribution is very unequal, the silver
passing almost completely into the zinc at temperatures not
far above the melting-point. Thus, at 540°, the partition-
coefficient is about 300, and is fairly independent of the
concentration.^ This fact is the basis of Parkes's process for
the desilverization of lead, which was first devised by Karsten
in 1842, but only brought into practical use some years later.
The argentiferous lead is melted, and small quantities of zinc
are added and thoroughly mixed by stirring. When cooled to
the freezing-point of the layer rich in zinc, a solid scum
separates, and, being lighter than lead, accumulates at the
surface, and is readily removed. The silver-content of the
residual lead may be reduced in this way to 0*0003 V^^ cent.,
whilst the upper layer contains as much as 25 per cent,
of silver. This indicates a very high partition-coefficient
near 400°.

As an example of a ternary system which has been more
fully worked out, we may take the alloys of silver, lead and
copper.' The space- model is illustrated in elevation and in
projection on the base in Fig. 41, the divisions of which
represent atomic percentages. The diagrams of the three
binary systems have been given previously (silver-lead. Fig. 8 ;
silver-copper, Fig. 23 ; copper-lead. Fig. 38). The freezing-
point surface of the ternary system is seen in the lower part of
the figure to be made up of three surfaces, one of which,
however, representing the separation of pure lead, is of very

' G. J. Petrenko, Zeitsch. anorg. Chem., 1906, 48, 347.

^ G. N. Potdar, 7. Coll. Set. Tokyo, 1908, 35, ix. Earlier measure-
ments were made by C. R. Alder Wright and C. Thompson, Proc. Roy.
Soc, 1890, 48, 25. Potdar's results indicate a much higher coefficient
than those of Wright and Thompson. The equilibrium diagram of the
ternary system has been determined by R. Kremann and F. Hofmeier,
Monatsh., 191 1, 32, 563, 597.

' K. Friedrich and A. Leroux, Metalliirgie, 1907, 4, 293.

92 METALLOGRAPHY

small area compared with the two others. The crystals
separating on the surface I. consist of a solid solution of
silver in copper. The alloys within the small area bounded
by a dotted line in the neighbourhood of Cu have a second
arrest during cooling, due to the separation of the mother-liquor
into two layers, and consequent crystallization of copper-silver
crystals at a constant temperature. The fact that the dotted
curve extends so little into the triangle is a proof that the
presence of silver greatly increases the miscibility of copper
and lead. Within the area I. many of the alloys must begin
their crystallization at a constant temperature, owing to the
formation of two liquid layers, but it was not determined how
far this extends into the triangle. Marked liquation was
observed in many of the alloys after cooling.

The solid solution II. contains silver with both copper and
lead. The dotted curve in the neighbourhood of Ag represents
the approximate limit of saturation of this solution. The line
DE is the boundary between the solid phases I. and II., and
along it the binary eutectic separates. The third area,
FEGPb, is that of crystallization of lead. It is separated
from the areas of I. and II. respectively by the binary eutectic
lines FE and EG, meeting in the ternary eutectic point E, the
co-ordinates of which are

Ag" 3-8)

Cu I '6 V atomic per cent. Temp. = 302'5°

Pb 94-6 j

The form of the diagram therefore resembles that of the
lead-tin-bismuth series in having three slooping eutectic valleys
meeting at a ternary eutectic point, only in this example the
point of intersection lies very near to the angle of the triangle.

Cu 1084°

Ag 962°

Cu

Fig. 41. — Silver, copper, and lead.

CHAPTER VI
PRACTICAL PYROMETRY AND THERMAL ANALYSIS

The apparatus required for the practical conduct of a thermal
analysis comprises (i) a means of heating and cooling the
alloy under examination, and (2) a means of measuring the
temperature of the alloy from time to time. Both these
requirements may be met in a great variety of ways, many
types of furnaces, and also of pyrometers, having been designed
and employed in investigations of this kind. Only such forms
will be described here as have shown their utility in practice.

Furnaces

A furnace intended for use in metallographic work must
fulfil certain conditions, which greatly restrict the number of
suitable types. The most important of these conditions are — ■

1. The distribution of temperature in the interior of the
furnace must be sufiSciently uniform to ensure that all parts
of the specimen under examination are sensibly equal in
temperature.

2. The temperature of the interior must be capable of
continuous variation throughout a considerable range, and it
must be possible to make the rate of cooling (or in certain
cases of heating) so slow that time is allowed for the attain-
ment of sensible equilibrium in the alloy.

3. It should be possible to heat the alloy out of contact
with gases capable of acting on it chemically or of dissolving
in it to a notable extent.

Gas and electric current are the only sources of heat which
94

PRACTICAL PYROMETRY

95

are sufficiently under the control of the experimenter to come
into consideration for such a purpose. When larger quantities
of alloys have to be prepared, however, it is more convenient
and economical to heat the crucible in a wind furnace fed with
coke, the product being either poured in a molten state into
smaller crucibles, or cast into ingots, which are subsequently
divided by sawing or breaking.

A gas furnace for metallography should be heated by means
of Bunsen burners, the horizontal burners with gauze or grid
generally proving unsatisfactory after they
have been in use for some time. A
large, solid flame of uniform temperature
is desirable, and this is only given by
the best forms of Bunsen burner. The
most suitable burner for the purpose,
especially when a temperature as high
as the melting-point of copper is re-
quired, is the M^ker burner, shown in
Fig. 42. The burner is of brass, with
very large holes for the admission of air,
so that the air supply is sufficient for
complete combustion. Firing back is
prevented by a deep nickel grid, the
openings of which form a series of tubes
2 mm. square and 10 mm. deep. The
large solid flame is of practically uniform
temperature throughout, there being no
inner cone of cooler gases as in the
ordinary Bunsen flame. The furnace
shown is intended for melting small quantities, it being
possible to melt copper in a short time. Cooling is too
rapid for the taking of satisfactory cooling curves, and further
provision against excessive loss of heat by radiation must be
made. The simplest contrivance for this purpose is composed
of two concentric cylinders of asbestos card, surrounding the
burner and crucible, the space between the two cylinders
being filled with fine sand or magnesia. A lid, made of several
thicknesses of asbestos card, and provided with a central hole

Fig. 42. — Meker
ciucible furnace.

96 METALLOGRAPHY

for the passage of the thermo-couple, is placed over the
cylinder as soon as the alloy is melted. Such a simple
arrangement is quite satisfactory when a determination of the
freezing-point is all that is required. For higher temperatures
the Fletcher concentric jet furnaces may be used, the blast
being supplied by a rotatory blower.

The additional convenience afforded by electric furnaces
is, however, so great that this form is to be preferred wherever
electric current is available. Of the types of electric furnace
which occur in commerce, neither the arc nor the induction
furnace is suitable for metallographic work, and the forms of
apparatus to be described all depend on the heating effect of
a current passing through a resistance. The resistance employed
may be —

1. A hehx of metallic wire or foil, wound on a non-
conducting tube ;

2. A tube of conducting material, through which a heavy
current of low voltage is short-circuited ;

3. A mass of granulated material of comparatively low
conductivity, loosely packed between terminal blocks of metal
or carbon.

4. A rigid helix of conducting carbon.

The majority of laboratory furnaces are of the first type.
In the older patterns, a helix of platinum wire was wound on a
porcelain tube. This had the disadvantage that the wire, if
thin enough to use current at the ordinary voltage, quickly
burnt out, whilst at high temperatures the porcelain was liable
to disintegrate and crack, owing to the very local character
of the heating. This difficulty is overcome in the Heraeus
furnace by using a ribbon of thin platinum foil in place of wire.
The temperature of the foil is never greatly different from that
of the tube, and local overheating is avoided. The helix
should be wound so closely that adjacent turns are separated
by the shortest possible distance, and they are kept in place by
an asbestos ribbon. The tube and foil are enclosed in a thick
insulating layer of magnesia, and an outer casing of asbestos
bound together by metal bands. The external appearance of
such a furnace, adapted for use in a horizontal or vertical

PRACTICAL PYROMETRY 97

position, is shown in Fig. 43. When used vertically, the
crucible is supported at a suitable height on a column of
porcelain or fireclay, a cap being placed over the top of the
tube to hinder the production of air-currents. When used
horizontally, as in annealing, it is desirable to close the ends
with mica .windows.

A platinum resistance furnace of this type may be heated
for a short time to 1300-1400°. Continued heating at the

Fig, 43. — Platinum resistance furnace.

latter temperature causes destruction of the tube ; it is there-
fore inadvisable to use this pattern of furnace for temperatures
above 1200°. Tubesjof alundum, prepared from pure alumina,
are very suitable for high temperatures. For lower tempera-
tures, fused silica tubes may be used instead of porcelain, but
silica becomes a conductor at about 1000°. A variable
resistance should be. used in series with the furnace, and the
life of the heating tube and helix is considerably prolonged if
care is taken to raise the temperature gradually. An ampere-
meter is generally inserted in the circuit for convenience in

T.P.C. H

98 METALLOGRAPHY

regulating the current. On a 220-volt supply, a furnace of
this kind requires from 6 to 14 amperes, according to the
temperature.

For alloys of fairly low melting-point, and for annealing
purposes, it is possible to work with a less expensive heating
material than platinum. Nickel wire, wound on unglazed
porcelain and packed in, quartz, has been found useful up to
1000°.^ Nickel wire, however, gradually disintegrates in
use, owing to the escape of dissolved gases.^ Much better
results are obtained with an alloy of nickel "nichrome,"
which has the further advantage of a very low temperature-
coefficient.

The second type of furnace, in which the resistance is
formed by a tube enclosing the heating-space, is mainly
employed for very high temperatures, above those at which
the ordinary pattern of furnace can be used. It has the
disadvantage that it cannot be worked by means of the
ordinary supply of current, but requires a transformer. Two
materials have been employed for the construction of the tubes,
namely, carbon and iridium. Of these, iridium has the draw-
backs of high cost and volatility. With an iridium tube
120 mm. long, 197 mm. in diameter, and 0-4 mm. thick,
protected by a fireclay cylinder, the temperature may be raised
to 1600° in 30 seconds, using a current of 900-1000 amperes
at 3 volts.'

Carbon tubes, which may be employed instead of the costly
iridium, have the disadvantage of forming carbon monoxide
in contact with air, which it is practically impossible to exclude.
This gas may in some instances affect the results seriously, as
it is readily dissolved by many metals. The risk is minimized
by the passage of a current of nitrogen during the ex-
periment.

A somewhat original modification of the tube resistance

' J. Harker, Phil. Trans., IQ04, 203a, 343.

2 H. C. H. Carpenter, Coll. Researches Nat. Phys. Lab., 1908, 8,
259.

^ K. Friedrich, Metallurgie, 1908, 5, 703, see also W. C. Heraeus,
Zeitsch. angew. Chem., 1905. 18, 49.

PRACTICAL PYROMETRY 99

furnace was designed and employed in work at very high
temperatures, exceeding the melting-point of platinum.^ The
tube in this apparatus was constructed of the Nernst mass used
in electrolytic filament lamps, composed of a mixture of rare
earths. Such a mass, whilst non-conducting at the ordinary
temperature, conducts electricity when raised to a red heat,
the conductivity then increasing as the temperature rises. A
metallic balancing resistance is therefore necessary. The
heating tube is enclosed in an outer tube of porcelain or fire-
clay which serves as a support, the intervening space being
packed with pure zirconia. The porcelain tube is wound
externally with nickel wire, by means of which the preliminary
heating is effected.

In the third type of furnace, the resisting material has a
granular form. The substance employed may be carbon or
a mixture of carbon, carborundum, and silicates.'^ A furnace
of this kind, for use at temperatures of 800-1000°, is illustrated
in Fig. 44.' The material is packed in a cylindrical space
between the two concentric fireclay cylinders a and b, current
being led in by carbon poles through the two spaces c and d,
filled with granules. These chambers are so wide that the
temperature is not greatly raised. / is the crucible. The
cylinders a and b, and the fireclay wedge g, are removable, as
they are liable to crack in repeated use. In the pattern shown,
there is also a channel, packed with granules, in the removable
lid, in order to maintain a uniform temperature. The lid is
furnished with an opening, k, for the passage of the thermo-
couple. Such a furnace may be used on a 220-volt circuit.
The same author describes {loc. cit.) a furnace for high tempera-
tures (1500°)^ in which the heating space is a narrow channel
in a mass of Meissen fire-resisting material.

Granular resistatices have the marked disadvantage that
inequalities in the packing are difficult to avoid, leading to

» J. A. Harker, Proc. Roy. Soc, 1905, 76a, 235.
» A. Buss, Zeiisch. aiigew. Chem., 1905, 18, 239.
' K. Friedrich, Metallurgie, 1907, 4, 778. See also F. Mattonet,
Melallurgie, 1908, 5, 186 ; Deckert, ibid., 638.

lOO

METALLOGRAPHY

local inequalities of temperature, which often result in the
cracking of crucibles, etc. At high temperatures arcs are
liable to form between granules which are not in contact.
After repeated use, also, the granular material shakes down
into closer packing, with the result that the temperature
attainable fails. These furnaces have found greater favour
on . the Continent than in this country. The eificiency of

Horizontal
Section,

Fig. 44. — Furnace for granular heating material.

this type has been systematically examined by H. Harkort,'
who finds that granulated carbon may be used in place of
complex material, and that the finer, the granulation, the higher
the resistance. The furnace found by him to give the best
results up to 1700° has the construction shown in Fig. 45.
The carbon is packed between two carbon blocks, a, a, in a
cylindrical space formed by the hard fireclay cylinder b and

' Metallurgie, 1907, 4, 617.

PRACTICAL PYROMETRY loi

the mass of compressed kaolin c. The outer packing is
composed of kieselguhr, held in place by an iron casing.
The carbon blocks are only in
contact with the granulated
carbon for the lower half of
their length, the upper part
being embedded in kaolin.
The carbon packing is con-
tinued above this point, in
order to form a reducing layer,
at a lower temperature than the
main mass. A double fireclay
lid is used. The consumption
of current is about 70 anipferes
on a iio-volt circuit.

Whatever construction of fur-
nace be adopted, precautions
must be taken against loss of
heat by conduction and radia-
tion. The furnace should be
enclosed in an outer non-con-
ducting double casing, which
may be conveniently built up
of asbestos card, the space
being filled with magnesia or

kieselguhr. The plan has also been adopted of enclosing
the whole furnace in a large water jacket, in order to
eliminate irregularities in the temperature of the environ-
ment.^

In taking a cooling curve, it is usual to shut off the current
when the alloy has reached a temperature sufficiently above
its melting-point, and then to allow cooling to take place by
radiation. This method has been modified in some recent
work on fused salts,^ and there are some advantages to be

Fig, 45.— Granular carbon
furnace.

• W. Rosenhain, J. Inst. Metals, 1909, 1, 245.

" W. Plato, Zeitsch. physikal. Chem., 1906, 55, 721. A similar
arrangement, liquid resistances being used to reduce the current, has been
employed by A. Portevin, Rev. de Mitallurgie, 1908, S, 295.

102 METALLOGRAPHY

anticipated from an application of the improved method to
metallography. Instead of stopping the supply of current
instantly, Plato adopts the plan of reducing the current in a
continuous manner from the maximum value to zero. A
Heraeus furnace is used in a vertical position, the crucible
being supported on a vertical porcelain column. The current,
obtained from a battery of accumulators giving a tension of
220 volts, passes through a resistance consisting of a nickel
wire wound on a porcelain cylinder, the other contact being a
spring pressing on the wire. The cylinder is slowly rotated
by means of a geared motor, and the resistance of the circuit
is thus gradually increased at any desired rate, producing a
gradual and regular diminution of the temperature of the
furnace. A cooling curve taken in such a furnace is
practically a straight line so long as no change involving the
development of heat takes place. The portion of the curve
representing the cooling of the liquid is also practically in a
line with that representing the cooling of the solid, and the
change in rate of cooling brought about by freezing is thus
particularly well marked, a fact which is of importance when
dealing with the theoretical interpretation of the curves (see
p. 124).

Provision should always be made, by means of fireclay or
porcelain plugs with inlet and outlet tubes, for the passage of
a current of gas during melting and cooling, in order to prevent
oxidation. Nitrogen is the most generally suitable gas for
this purpose, but if taken from a cylinder, it should be passed
through a vessel containing sticks of phosphorus, or through
a tube containing heated copper, in order to remove the
oxygen which is always present. The use of nitrogen is, of
course, excluded when metals capable of forming nitrides, such
as magnesium, are present. Hydrogen may also be used, and,
in certain cases, carbon dioxide. The last-named gas is
decomposed by magnesium. The current of gas is led in by
a tube of Jena glass or Marquardt porcelain, according to the
temperature required, the tube terminating just above the level
of the alloy. Volatile metals, such as zinc, arsenic, or cadmium,
may pass through the porcelain walls of a furnace in the form

PRACTICAL PYROMETRY 103

of vapour, and may attack the platinum used as a heating
material. Manganese is particularly' difficult to deal with at
high temperatures for this Ireason, as its vapour readily passes
through a wall of Marquardt porcelain.' It is necessary there-
fore to shield the porcelain by means of an inner iron tube
wrapped with asbestos.

It is of great advantage to have a ready means of heating
and melting alloys in a vacuum, since no gas is absolutely
undissolved by molten metals. An electric vacuum furnace
has been successfully employed in the investigation of electro-
lytic iron, from which it was necessary to remove occluded
gases.^ This furnace is illustrated in Fig. 46. The heating
device is a carbon helix, constructed by cutting a helical slit
in a tube of hard carbon, 300 mm. long and 40 mm. internal
diameter, the thickness of the tube being 6'5 mm. and the
width of each turn 11 mm. This helix stands in a wider
carbon tube, from which it is insulated by porcelain collars.
Both tubes fit into a brass cylindrical vessel, to which glass
flanged ends are luted, the current being led in by wires
passing through glass side arms. The crucible is supported
on a carbon tube, carried by an iron rod. The brass vessel
can be exhausted, and is contained in an outer metallic vessel
serving as a water-jacket.'

> F. Wiist, Metallurgie, 1909, 6, 3.

« A. Miiller, MetallurgU, 1909, 6, 145. A simpler form of this
furnace was used by P. Oberhoffer (jbid., 1907, 4, 427), in an investigation
of the specific heat of iron.

" Other forms of vacuum furnace are described by W. J. Forsythe,
Astrophys. J., 191 1, 34, 3S3 ; and by J. C. W. Humfrey, Iron and Steel
Carnegie Schol. Mem., 19 1 2, 4, 80; the latter being a very simple
form.

I04

METALLOGRAPHY

Crucibles

For work at moderate temperatures, either fireclay or
plumbago crucibles may be conveniently used, the latter con-
sisting of a mixture of graphite and fireclay, which resists
changes of temperature, and is free from porosity. Jena glass

Thermo
Couple

MagnesiO'
tube

Fig. 46. — Electric vacuum furnace with caibon spiral.

tubes have been used extensively by some workers, and are
convenient when working with amalgams. They are, however,
necessarily of small capacity, and it must be emphasized that
cooling curves, taken with a quantity of only 10 grams or so

PRACTICAL PYROMETRY 105

of alloy, are worthless for the construction of the thermal
diagram. A quantity of at least 100 grams is required in
order that trustworthy results may be obtained, and crucibles
of this capacity should be used as far as possible. Fireclay
cannot be used at very high temperatures on account of its
softening, and it is further readily attacked by the layers of
slag formed by the oxidation of many metals, especially lead
and manganese, whilst the presence of reducing metals, such
as aluminium or calcium, in the alloy to be melted, leads to
the reduction of silicates, and the introduction of silicon as an
impurity into the alloy. Carbon crucibles, turned from a
block of artificial graphite, are often very convenient. Carbon
is, however, taken up by metals of the iron group, whilst the
presence of oxygen, which cannot be completely excluded
from ordinary furnaces, leads to the formation of carbon
monoxide, which is dissolved by many molten metals.

Hard porcelain, as made at the Berlin or Meissen works,
is a very useful material. Like fireclay, it is attacked by
those metals and slags which act chemically on silicates.
Magnesia crucibles, made from magnesia fired at a very high
temperature, are perhaps the most generally applicable to
alloys of high melting-point. They are somewhat fragile, and
require careful handling when hot. They are, moreover, ^
porous to certain metals, copper passing through them quite
readily.

A fireclay or plumbago crucible may be made more
resistant by brasquing, that is, by lining with a more resistant
material. Graphite or retort carbon, mixed to a stiff paste
with sugar syrup or tar, makes a good carbon lining j whilst
fused and ground magnesia, mixed with a little magnesium
chloride solution, may be used as a lining for many purposes.

The Berlin Porcelain Manufactory now manufactures
crucibles of spinell mass (2MgO,Al203), which is impervious
to carbon monoxide, and may be heated safely to 1700°.
Alumina, especially in the pure form known as alundum, has
also been used as a material for crucibles. Like magnesia,
alundum is highly porous. Carborundum and other materials,
including the very resistant pure zirconia, have been tried in

io6 METALLOGRAPHY

technical practice but have not yet come into general use in
laboratories.^

When an indifferent atmosphere is not employed, and air
is admitted to the furnace, it is generally necessary to cover
the surface of the alloy with a protecting layer, Pure sugar
charcoal or wood charcoal may be used at low temperatures,
graphite at very high temperatures. Carbon will not, however,
protect zinc or magnesium from oxidation, and its presence
must be avoided when metals capable of combining with it,
such as iron or nickel, are being melted. The following salts
are suitable for use as a protecting layer : borax (m.-p. 741°),
sodium chloride (m.-p. 820°, volatile at its melting-point),
anhydrous carnallite (KCl,MgCl2, m.p. c. 450°), potassium
cyanide (m.-p. 622°), barium chloride (m.-p. 960°),

Preparation of Alloys

Alloys ai-e generally prepared by fusing together their com-
ponents. When these are readily fusible, the operation presents
no difficulty, as it is sufficient to introduce the weighed metals
into a crucible and to heat until molten, when the mass is
thoroughly stirred to ensure complete admixture. It is not
possible to alloy metals of very different melting-points in this
way. If zinc and copper are heated together, the zinc is mostly
lost by volatilization and oxidation before the melting-point of
the copper is reached. In such a case the copper should be
melted first, and the zinc added to it. The zinc should be in
coarse pieces, and should be immediately pushed under the
surface of the copper, the mass being stirred with a fireclay
or carbon rod after each addition. As the first fragments of
zinc are added, the high temperature of the molten copper
causes some volatilization of zinc, and a white flame is produced,
white flakes of zinc oxide being formed. As the temperature
of the alloy is lowered by successive additions of zinc, the

' The properties of heat-resisting materials intended for exposure to
high temperatures have been studied by L. Baraduc-MuUer, Rev. dt
Metallurgie, 1909, 6, 700 ; S. Wologdine, ibid., 767.

PRACTICAL PYROMETRY 107

" flaring " becomes less marked. The loss of zinc, amounting
to about 2 per cent, when making up an alloy of 60 per cent.
Cu and 40 per cent. Zn on a small scale, must be allowed for
in weighing out the quantities of metal to be used.

Aluminium should be added in the same way as zinc, as if
melted alone it becomes covered with a thin, but very tenacious,
layer of aluminium oxide, which coats any solid metal on its
introduction and thus prevents alloying. When aluminium is
added to molten copper, a great rise in temperature is observed,
the alloy often becoming white-hot, in spite of the cooling due
to the introduction of the cold metal. This is not due, as
was once supposed, to the heat of combination of copper and
aluminium, but to combination of a part of the aluminium
with the oxygen which is always present in commercial copper,
and a further quantity of which is absorbed from the air by
the copper during melting. This activity of aluminiutfl in re-
moving oxygen from copper is often made use of in preparing
copper alloys. The presence of oxygen in these alloys is un-
(lesirable, as the metals added form oxides which remain
entangled in the molten mass, diminish its fluidity, and cause
heterogeneity in the ingot.' The addition of aluminium brings
about the reduction of other oxides, and the formation of
aluminium oxide, which separates from the alloy with greater
facility. Magnesium is used in a similar manner. Phosphorus
and silicon, which are excellent deoxidizers, are commonly
added in the form of alloys with copper or some other con-
stituent of the alloy to be prepared. Thus phosphorus is
added in the form of 10 per cent, phosphor-copper, a com-
mercial product, I per cent, of which, equivalent to o'r per
cent, of phosphorus, is usually sufficient to deoxidize copper
under ordinary conditions. The quantity added should be
sufficient to remove the whole of the oxygen, leaving none,
or at most a trace, of the deoxidizer in the final product.
Iron and its alloys are frequently deoxidized with ferro-sllicon.

' E. Heyn and O. Bauer, Zeitsch. anorg. Chem., igoj, 45, 52, have
investigated the influence of oxygen on the alloys of copper and tin. The
stannic oxide crystals formed separate with difficulty from the molten
alloys.

io8

METALLOGRAPHY

Alloys with arsenic are troublesome to prepare, on account
of the great volatility of this element, but by employing an
electric furnace with carbon tube resistance, and heating very
rapidly to a high temperature, alloys with platinum containing
from 27 to 28*3 per cent, of arsenic have been prepared.^ It
is generally preferable to make a rich alloy of the metal in
question with arsenic, and to dilute this by melting with
successive larger quantities of the second component. It is
rarely possible to examine the entire series up to arsenic, as
the richer alloys dissociate when heated under atmospheric
pressure. The rich alloy is conveniently prepared by heating
the oxide of the metal with a considerable excess of arsenic.
This plan has been adopted in studying the alloys of arsenic
with nickel ^ and with cobalt.^

The preparation of a rich alloy, which is subsequently
melted'with further quantities of one of its components, is a
convenient means of avoiding loss by oxidation and burning,
as the dilution with the second metal commonly takes place
by quiet fusion. Rich alloys of aluminium or zinc with copper
may be prepared and analysed, and by adding the calculated
quantity of copper other alloys of the series may be accurately
prepared. A ternary alloy of copper, nickel, and zinc, known
as German silver, is thus prepared by melting together alloys
of copper and nickel, and copper and zinc.

Metal.

Boiling-point.

Metal.

Boiling-point.

Mercury

357°

Aluminium .

1800°

Cadmium .

780°

Manganese

1900°

Zinc . . .

920°

Silver .

1955°

Magnesium .

1120°

Chromium

2200°

Bismuth

1420°

Tin ...

2270°

Antimony . .

1440°

Copper

2310°

Lead

1525°

Iron

2-15°°

The relative volatility of some of the principal metals may

• K. Friedrich and A. Leroux, Metallurgies 1908, 6, 148.
' K. Friedrich and F. Bennigson, ibid., 1907, 4, 200.
' K. Friedrich, ibid., igo8, 5, 150.

PRACTICAL PYROMETRY 109

be seen in the table on p. 108, which gives the boiling-points
under atmospheric pressure. ^

Arsenic sublimes without melting. Its sublimation tempera-
ture under atmospheric pressure is in the neighbourhood of 450°.

The metals iron, chromium^ manganese, tungsten, moly-
bdenum, vanadium, etc., and their alloys, if prepared by re-
duction in the laboratory, form powders which it is difficult
to fuse together into a coherent mass. Reduction by the
electric arc yields massive metals and alloys, but these always
contain considerable quantities of carbon derived from the
electrodes. Commercial ferro-silicon and ferro-manganese
contain carbon for this reason, and their use in preparing
alloys necessarily results in the introduction of this element,
the presence of which modifies the equilibrium considerably.
The great reducing power of aluminium at high temperatures
has been utilized in what is known as the Thermit process to
produce metals and alloys free from carbon.^ The metallic
oxide, or mixture of oxides if an alloy is to be prepared, is
ground and mixed with the necessary quantity of aluminium
for complete reduction and "introduced into a crucible. The
reduction proceeds with great development of heat, and it is
only necessary to produce a local elevation of temperature to
initiate the reaction. This is accomplished by means of a
small quantity of a mixture of fine aluminium powder and
barium peroxide, which is placed in a heap on the surface of
the thermit mixture, and ignited with a piece of magnesium
ribbon or of thread impregnated with potassium nitrate. The
reaction begins immediately, and propagates itself very rapidly
through the mass, so that a few seconds suffice to bring the
whole contents of the crucible to a white-hot molten state.
The metal or alloy may be poured into a mould, the liquid
slag, which consists of fused alumina, being collected separately.
The products usually contain some aluminium, but a high
" degree of purity may be attained. Alloys of aluminium are

' The values for the less volatile metals were obtained by means of an
optical pyrometer ; H. C. Greenwood, Proc. Roy. Soc, 1909, 82a, 396.

^ H. Goldschmidt, Annakn, 1898, 301, 19 ; Physikal. Zeilsch., 1902,
4, 166.

no METALLOGRAPHY

readily prepared by employing an excess of the reducing
metal. Alloys containing copper and other more fusible
metals may also be prepared ; but the reaction in such cases
is violent and even explosive, owing to the evolution of
metallic vapour, so that the reacting substances should be
mixed with alumina or other inert material.

Calcium, silicon, and magnesium have also been employed
in the thermit reaction, but can only find application in
exceptional cases. ^

The metallographist often has occasion to prepare castings
of alloys for the purpose of examining their physical properties.
Cast-iron moulds, divided longitudinally and held together
by a clamp, are useful for this purpose. The inner surfaces
should be rubbed with graphite and the mould warmed before
use. Simple sand moulds are easily prepared, a smooth
wooden rod being used as the pattern around which the
moulding sand is rammed. A sufficient head of metal must
be allowed, so that gas bubbles, etc., will collect in the upper
part of the rod, which is cut off and rejected. A short length
at the lower end should also be rejected, as it is liable to
contain sand swept down by the first rush of metal into the
mould. To guard against errors due to segregation in the
ingot, a small piece should be cut from the upper and also
from the lower end of the rod for microscopical examination
and chemical analysis.

Although alloys are generally prepared by fusion, there
are many other ways in which they may be produced.^ The
" cementation " of iron, its conversion into steel by heating in
solid carbon or carbonaceous matter, or in carburizing gases,
will receive consideration in dealing with the process of diffu-
sion. A similar process may be applied to other metals.
Thus iron and copper may be coated with zinc by heating in
zinc dust at a temperature of only 200^ a true alloy being
produced at the surface. The electrolytic deposition of metals
from mixed electrolytes also results in the production of

1 F. M. Perkin, Trans. Faraday Soc, 1907, 3, 115; F. M. Perkin
and L. Pratt, ibid., 179.

^ S. Cowper-Coles, Electro-Chemist and Metallurgist, 1903, 3, 828.

PRACTICAL PYROMETRY in

alloys,^ and this process finds technical application. Amalgams
may be prepared by the electrolysis of a salt of the required
metal, using a mercury cathode. A continuously acting
apparatus has been devised for this purpose.*

Pyrometers

Of the many types of instrument employed for the measure-
ment of temperature, very few are suitable for the accurate
determination of freezing- and transformation-points in metal-
lography. Mercurial thermometers are only available for those
amalgams which are liquid at or near the ordinary temperature,
and perhaps for isolated experiments on fusible alloys melting
at or near ioo°. They are not suitable for the determination
of transition points, on account of the difficulty of making
perfect contact between the bulb and the specimen under
examination. Thermometers having the stem above the
mercury column filled with nitrogen or carbon dioxide under
pressure, are obtainable with graduations up to 450°; their
employment at such temperatures, however, is very unsatis-
factory, owing to the uncertain character of the permanent
deformations produced in the glass bulb.

The available types of pyrometer, excluding the mercury
thermometer, are three in- number, namely, the thermo-electric
couple, the electrical resistance pyrometer, and the radiation
pyrometer. Of these, the first is by far the most important,
from the great range of temperature over which it may be
used, and the sensitiveness which may be imparted to it by a
suitable disposition of the electrical parts. It depends on the
measurement of the thermo-electric difference of potential
produced when a junction of two dissimilar metal wires is
heated or cooled, the other junctions being maintained at some
constant temperature. In practice, the two wires forming the
couple are welded together to form the hot junction, the cold
junctions being the points at which the ends of the couple wires
are joined to the galvanometer leads.

' F. Mylius and O. Fromm, Ber., 1894, 37, 630 ; L. Kahlenberg,
Electro-chem. Ind., 1903, 1, 201 ; S. Field, Trans. Faraday Soc, 1909, 5,
172.

* W. Kerp, Zeitsck. anorg. Chem., 1898, 17, 284.

112 METALLOGRAPHY

The metals used for thermo-couples are principally those of
the platinum group. Other couples, such as those of iron and
constantan or copper and constantan, develop a higher thermo-
electromotive force, but their use is restricted to low tempera-
tures. For measurements at high temperatures, it is necessary
that the metals forming the couple should be highly infusible,
and should also be unaffected by hot air. In both the couples
in general use, one wire is of pure platinum, the other being an
alloy of 90 per cent, platinum and 10 per cent, rhodium, or of
90 per cent, platinum and 10 per cent, iridium. The former
is the more often used. The Pt-Ptir couple has the higher
E.M.F. at a given temperature, but its use is restricted to
temperatures below 1400°, in spite of its infusibiUty, as iridium
is volatile, and the vapours cause contamination of the second
wire. When it is desired to measure temperatures above the
melting-point of platinum, the only couple available is one com-
posed of wires of iridium and an alloy of iridium and ruthenium
respectively, which has the disadvantage of being very brittle.
For such extreme temperatures, optical methods are generally
to be preferred. Whichever couple is used, the wires should
be thoroughly annealed before any measurements are taken,
and re-annealing at a high temperature is also desirable when
a couple has been long in use, as the pure platinum wire
readily absorbs gases.

The E.M.F. of a thermo-couple is not proportional to the
temperature. Accurate comparisons of couples with the gas
thermometer have been made,' and*it has been found that the
relation is most conveniently given in the form

in which E is the E.M.F. in microvolts, B is the temperature,
and a, b, and c are constants. A logarithmic formula, of the
form

log E = a log -{■ b

is applicable over a somewhat wider range.

1 L. Holborn and A. L. Day, Amer. J. Sci., 1899, [iv.] 8, 165 ; J. A.
Harker, Phil. Trans., 1904, 203a, 343.

PRACTICAL PYROMETRY 113

In practice, however, the temperature is best found from
the E.M.F. by a graphical construction. If the couple is pro-
vided with a certificate, a table of the observed E.M.F.'s at a
number of different temperatures is there given, and it is also
desirable to make some determinations of the freezing-points of
pure metals. The fixed datum-marks thus obtained are used
for the construction of a curve, from which the temperature
corresponding with a given reading is obtained with ease. It
must be remembered that extrapolation beyond the highest
point actually determined is attended with great uncertainty,
and the equations expressing the relation between E and 6
become less and less trustworthy as the temperature increases.

The above formulae are based on the assumption that the
cold junctions, that is, the points at which the wires of the
couple make contact with the leads, are at 0°. This condition
is not always convenient in practice, and for ordinary purposes
it is better to use a water-bath at a constant temperature in the
neighbourhood of 15°. The arrangement shown in Fig. 47 is
suitable for this purpose, the junctions being enclosed in glass
tubes closed with paraffined corks, immersed in a water-bath
provided with a thermometer. A correction has to be made
for the temperature of the cold junctions. Supposing this to
be 15°, it is necessary to add a correction, amounting to 13 "3°
when the hot junction is at 100°, 11 '4° at 200°, 8"8° at 400°,
diminishing to 7'5° at 900°. Intermediate values may be found
by interpolation.

When accurate measurements are being made, the necessity
of applying a correction is avoided by filling the outer vessel
in Fig. 47 with broken ice, so that the cold junctions are kept
constantly at 0°. Some observers prefer to maintain this
junction at 100°, by enclosing it in an apparatus in which water
is boiling, a condenser being attached.

The wires constituting the couple should be o"6 mm. thick
and 60-100 cm. long, in order that the cold junctions may be
at a safe distance from the furnace. To insulate one wire from
the other, it is enclosed in a capillary tube, which may be of
quartz or of porcelain, or, for very high temperatures, of mag-
nesia, whilst the whole is enclosed in an outer protecting tube,

T.P.C. I

114

METALLOGRAPHY

as described above. Outside the furnace, the wires are
separated by being threaded through doubly bored fireclay

PlaUnum-

Copper

W

PlatiTvwm
-rhodiu,rrv

Copper

rods about 6 cm. in length, which
may be obtained from the dealers
who supply the thermo-couples.
The attachment to the galvano-
meter leads is made by means
of small double binding screws.
The leads themselves are of in-
sulated copper wire.

In the simplest method of
measuring the thermo-electro-
motive force developed by the
couple, the leads are connected
directly with a galvanometer,
such as a Siemens and Halske
millivoltmeter, on the scale of
which 1° C. = o'ooot volt, at
iooo°, a platinum platinum-rho-
dium couple being used. Such instruments are often provided
with a scale graduated directly in temperatures, but the readings
should be carefully calibrated from time to time by determining
the freezing-points of jjure metals. The following freezing-
points are in common use for calibration : — '

KJ-

Fig. 47.— Cold junction.

Metal.

Atmosphere.

Freezing-point.

Metal.

Atmosphere.

Freezing-point.

Tin . .
Cadmium
Lead . .
Zinc . .
Antimony

Air
}i
i»
»f

Carbon
monoxide

)}
Hydrogen

232°
320°±o-3°

327-4°
4i8-2±o-3°
629-2±o-5°

658-o±o-6°

96o'o±o-7°

io62-4±o-8°

io82-6±o-8°

i452-3±2-o°

Cobalt .
Palladium
Platinum .

Hydrogen
Air

»i
(optical)

i489-8±2-o°
i549-2±2-o°
1752±5°
I755±S°

Aluminium
Silver. .

Mineral.

-

Melting-point.

Gold . .
Copper .
Nickel .

LijSiOj .
Anorthite

I20I±I° <

iS49'S±2-o°

' The figures are mostly taken from A. L. Day and R. B. Sosman,
Carnegie Inst. Publ., 191 1, No. 157.

PRACTICAL PYROMETRY 115

together with the boiling-points of water (100°) and sulphur
(4447°). The freezing-points of silver and copper are much
lower in presence of air.

The accuracy of the millivoltmeter type of instrument is
not very high. It is suitable for measuring annealing tempera-
tures or for determining freezing-points when very high accuracy
is not required, but it is not satisfactory as a means of investi-
gating a series of alloys. The accuracy may be largely increased
by the use of a suspended coil instrument with mirror, the
readings of the spot of light reflected on a scale being taken.
It is not worth while, however, to increase the sensitiveness
of the galvanometer very greatly, as the direct method of
measurement is subject to certain errors, depending on the
change of resistance of the couple itself with temperature,
which limit its accuracy.

The thermo-couple may be made self-recording, either by
causing the spot of light to fall on a sheet of sensitized paper
wound on a drum rotated by clockwork, as in the well-known
recording instrument of Roberts-Austen,' or by means of a
mechanism depressing an inked thread against paper at regular
intervals.^

For greater accuracy, the E.M.F. should not be read
directly, but should be balanced against that of a standard
cell, a potentiometer being used to effect the balance. A
special construction of the potentiometer is required, the
E.M.F, to be measured being very small, whilst it is essential
that readings should be taken in rapid succession. A null
method does not provide the required rapidity of manipulation,
and the device, due to Stansfield,' of compensating the greater

' Fifth Rep. Alloys Research Committee, 1899.

* It is difficult to ensure a perfectly steady motion of the photographic
or other paper, hence it has also been proposed to use a stationary photo-
graphic plate, the image being displaced by a mirror moved at a uniform
rate (S. Wologdine, Rev. de Mitallurgie, 1907, 4, 552).

» A. Stansfield, PMl. Mag., 1898, [v.] 46, 59. Another form of this
installation, arranged for photographic recording, has been extensively
used for metallographic purposes by the Russian investigators. See N. S.
Kurnakoff, Zeitsch. anorg. Chem., 1904, 42, 184. See also W. P. White,
Phys. ReV; 1907. 25, 334.

ii6 METALLOGRAPHY

part of the E.M.F., leaving a small outstanding part to be
read on a scale in the usual manner, is generally adopted. A
large standard cell or a small secondary battery il used to
produce the balancing E.M.F., and a sensitive Desprez-
D'Arsonval galvanometer is used for reading. The sensitive-
ness may be so chosen that the whole range of the scale, some
60 cm. long, represents only 10° or 15°. When the spot of
light has reached the end of the scale, the resistance of the
potentiometer is altered, so reducing the balancing E.M.F.
and bringing the spot back to the beginning of the scale.
This plan is only suitable for slow cooling, as otherwise the
time required for the oscillations of the galvanometer to die
down may cause the loss of a reading. It is also affected by
zero-creep, to which all suspended-coil galvanometers are
liable, but with suitable precautions very accurate results may
be obtained by its means.

In the various difference methods,^ the difference of
temperature between the specimen under examination and the
furnace is measured at frequent intervals or continuously.
Since the temperature of the furnace is not so readily or so
accurately determined as that of a body enclosed in it, Roberts-
Austen proposed the device of placing a neutral body, such as
a mass of platinum, near to the body under examination and
under the same conditions of temperature as it.^ The neutral
body is one which gives a perfectly regular cooling curve;
it 'must not, therefore, present any discontinuity of thermal
properties within the range to be observed. Platinum is
obviously the most suitable substance for the purpose, although
nickel may be used successfully for ranges of temperature
which do not include its transformation point. The neutral
body should be as nearly as possible of the same shape and
size as the body investigated, in order that they may cool with

' Commonly called differential methods. . Since, however, true
differential methods of plotting, involving the differential coefficient
of a function, are now frequently adopted, it is better to use the
term difference method for one in which differences of temperature are
measured.

" Fifth Report, Alloys Research Committee, 1899.

PRACTICAL PY ROME TRY

117

equal rapidity. Tiie diflference in specific heat of the two
metals causes a departure from uniformity in cooling ; but this
error may be to some extent eliminated in plotting the curves,
as described below.

In Roberts-Austen's method of working, the two thermo-
couples, placed in holes drilled in the specimen and the neutral
body respectively, are connected so that the E.M.F.'s produced
tend to neutralize each other, that is, the platinum wires of the

Fig, 48. — Potentiometer and diflference method.

two couples are connected together, the other wires being
connected with the galvanometer. So long as the bodies are
cooling at the same rate, the galvanometer does ifot indicate
the passage of any current ; but any development of heat,
caused by a polymorphic change in the specimen, gives rise
to a difference of temperature (S-5i) between the two, bodies,
so that the galvanometer coil is deflected. The temperature
6 of the specimen is observed at the same time, either by a
separate thermo-couple, or by the shunting of one couple
through a second galvanometer. The former arrangement is
more suitable for accurate work, and is illustrated diagram-
matically in Fig. 48.1

The specimen S and the neutral body N are contained in the
cylindrical electric resistance furnace A. The thermo-couple
B is connected directly with the potentiometer, whilst the
two couples C and D are so connected that their platinum-

' H. C. H. Carpenter and B. F. E. Keeling, y. Iron Steel Inst., 1904,
i. 224.

ii8 METALLOGRAPHY

rhodium wires are joined, and their platinum wires lead to the
galvanometer d, the deflections of which measure fl-^i.
Kj, Kj and K3 are cold junctions, maintained at 0°. The
balancing E.M.F. is furnished by the accumulator P, and is
checked from time to time by means of the standard cells Q,
of I -019 volts each. Rj and Rg are resistance boxes, of which
Ri can be used for balancing Q against P until the deflection
of the galvanometer G3 becomes zero, R^ having the resistance
1 01 9 ohms. The potentiometer consists of four sets of nine
coils each, arranged in two rows, and connection may be made

Fig. 49. — Saladin's difference method.

between them at any two points by the plugs E and F. The
coils in the left-hand half are- of 2 ohms each, and those in the
right-hand tialf of the potentiometer o'2 ohm each. Each
02 ohm between E and F then corresponds with about 400
microvolts. The thermo-couple being connected by the key,
balance is found to the nearest 400 microvolts, and the out-
standing E.M.F. is read off by means of the galvanometer G2,
which is a sensitive suspended coil instrument, with mirror,
giving a deflection on the scale of 8 mm. for 1°, so that very
small changes in Mj may be read off.

The reading of the time is eliminated altogether in the
ingenious recording method of Saladin,^ by which fl-^i is
plotted directly against 9. The photographic plate on which
the record is made is stationary, and the beam of light is
reflected by the mirrors of both galvanometers. The arrange-
ment is shown in its most useful form in Fig. 49. The

' Assoc, intern. MHhodes WEssais, February, 1903, described by H.-
Le Chatelier, Rev. de Mitallurgie, 1904, 1, 134.

PRACTICAL PYROMETRY 119

specimen, neutral body and thermo-couples are arranged as
before, and their lettering corresponds with that of Fig. 48.
The two galvanometers, Gi and Ga, are placed side by side on
a common base, rendered as free from vibration as possible by
layers of lead and felt. A beam of light from the Nernst lamp
H, issuing through a narrow slit, passes through the colour-
filter F, and is rendered parallel by the lens Li before falling
on to the mirror of the sensitive galvanometer Gi, the deflec-
tions of which are proportional to fl-5i. These deflections
are naturally in a horizontal plane. The reflected beam now
enters a totally-reflecting prism P, inclined at an angle of 45*.
The horizontal displacements of the ray are in this way con-
verted into vertical displacements before reaching the second
galvanometer, G2, which measures the temperature, h. The
mirror of G2 must be of sufficient height to receive the reflected
ray from Gi at its maximum displacement. The beam of
light leaving G2 has now a double motion impressed on it, a
vertical one due to Gi and a horizontal one due to G2. It is
focussed by the lens Lg on to the plate J, where it records
itself as a fine curve, the ordinates of which are the values of
fl-flj and the abscissae those of 9. If a record of the time is
also required, this may be effected, as proposed by Le Chatelier,
by interposing a toothed wheel (a disc of cardboard into the
edge of which pins are fastened at regular intervals) in the
path of the ray between H and Lj. This wheel is rotated by
the mechanism of a small clock, and produces short interrup-
tions in the curve at definite intervals of time.

It is necessary that G2 should be free from zero-cfeep,
which gives rise to errors in the temperature at which the
development of heat occurs. The same apparatus has been
used for recording temperature-time curves with a single
thermo-couple.^ This is connected with G3, whilst the deflec-
tions of Gi are made proportional to the time, a cell being
connected with the galvanometer through a very high resist-
ance (10,000 ohms). This circuit is divided, one branch
including the galvanometer, whilst the other includes a fine
platinum wire dipping into mercury. By means of a regulating
' H. Harkort, MHallurgie, 1907, 4, 639.

t2o METALLOGRAPHY

water-flow, the level of the mercury can be progressively'
altered, thus varying the resistance of the circuit, and causing
a continuous displacement of the galvanometer mirror.

In the electrical resistance pyrometer, advantage is taken
of the change of resistance of a platinum wire with tempera-
ture.' This method of pyrometry is admirably adapted to work
of great precision, since very minute differences of temperature
may be determined by its means. It is, however, less suited
to the work of thermal analysis than the thermo-couple, on
account of the greater difficulty of reading, and also of the com-
paratively large mass of the thermometer portion of the instru-
ment, which renders it difficult to follow rapid fluctuations of
temperatures. The method is interesting as having been em-
ployed by Heycock and Neville in their investigations of the
freezing-points of metals and alloys, undoubtedly the most
accurate series of measurements of this kind ever made.' The
end portions of the freezing-point curves, which are of great
importance as giving information as to the molecular condition
of alloyed metals, were, in particular, determined by these
investigators with great precision.

In the original form of instrument the platinum wire is
wound on a mica frame enclosed in a porcelain tube. In a
later form the wire is wound on a quartz rod, which is then
slipped into a thin quartz tube, the latter being finally fused on
to the rod, so embedding the wire in quartz. This gives a very
narrow bulb, occupying little more space than a thermo-couple.
The resistance is measured by a Wheatstone bridge arrange-
ment, which may be made self-registering.

Several forms of optical and radiation pyrometers have
been employed for work at very high temperatures. None of
these are suitable for the determination of cooling curves, but
they find application in metallography in the measurement of
melting-points of substances fusible with difficulty, and in the
determination of furnace temperatures in quenching experi-
ments, etc. The optical pyrometer depends on the comparison

■ H L. Callendar, Phil. Trans., 1887, 178a, 161 ; H. L. Callendar
and E. H. Griffiths, ibid., 1891, 182a, 119.

» C. T. Heycock andF. H. Neville, Trans. Chem. Soc, 1895, 67, 160.

PRACTICAL PYROMETRY 121

of the light emitted by the specimen with that from a standard
source, only rays of a particular wave-length being selected for
comparison. In the Holborn-Kurlbaum instrument ^ an electric
glow-lamp is used as the standard source, the current passing
through it being adjusted until the image of the filament just
disappears on the field illuminated by the radiating body, a
red glass being interposed to select a certain group of radia-
tions. With the cuprous oxide glass used by Holborn, the
radiations transmitted have their centre at A, = o"643ju,. The
relation between the current passing through the filament and
the temperature, assuming the emitting object to have the
properties of a black body, is given by a parabolic formula.
The intensity I of the monochromatic radiation varies with the
temperature, according to Wien's law —

I = QX-h'^

where T is the absolute temperature, and C and c are constants.
Very high temperatures may be measured by using a tungsten
or other metallic filament as the standard source. The instru-
ment is readily calibrated by observing a series of melting-
points of pure metals.

In the Wanner pyrometer, which is extensively used in
metallurgical laboratories, coloured glasses are avoided, and
the lamp is run on a constant current. The beams of light
from the object and the standard lamp are decomposed by
a direct vision prism, and brought together in a photometer.
One or the other is weakened by turning a pair of polarizing
prisms until the intensities are equal.

The Holborn-Kurlbaum pyrometer has been utilized for
the determination of melting-points of metals belonging
to the iron group.^ A blackened brass cyUnder was used,
enclosing an electrically heated strip of thin platinum, on
which the fragments of metal could be placed. The vessel
was filled with hydrogen to prevent oxidation. The pyrometer
was directed on to the platinum, and the temperature read at

' L. Holborn andF. Kurlbaum, Ann. Physik., 1903, [iv.] 10, 225 ; L.
Holborn and S. Valentiner, ibid., 1907, [iv.] 23, i.

'^ G, K. Burgess, Bull. Bureau Standards, Washington, 1907, 3, 345.

122 METALLOGRAPHY

the moment of fusion. Two corrections were necessary, one
depending on the selective emission of the platinum for the
ray selected, X = o'66/x, and the other on the loss of light by
reflection and absorption in passing through the mica window
provided for observation.

The melting-points found by this method — namely, iron,
1505°; chromium, 1489°; cobalt, 1464°; nickel, 1435"; man-
ganese, 1207° — are in good agreement with the freezing-points
found by other observers.

The Fdry radiation pyrometer measures the heat radiation,
which is related to the temperature by the Stefan-Boltzmann
law, according to which

E = k{T* - To*)

E being the energy radiated, T the absolute temperature of the
body. To that of the surroundings, and k a constant.

The instrument is set up like a telescope at some distance
from the heated object. The tube contains a concave mirror,
by means of which the radiations are focussed on to a small
thermo-couple, the E.M.F. of which is read by a galvanometer
in the usual way. The temperature readings obtained are,
within certain limits, independent of the distance of the
pyrometer from the object. It is only necessary that the
object should be near enough for its image to overlap the small
thermo-couple. If this condition is fulfilled, moving the pyro-
meter towards the object enlarges the image, but the part of it
that falls on the couple is of the same size as before.

Radiation pyrometers only give the correct temperature if
the emissive power of the object is that of a black body.
This is not often the case, but " effectively black " radiations
are emitted by surfaces in the interior of a uniformly heated
space, so that the inner wall of a muffle, etc., may be con-
sidered as having the emissive power of a black body. When
taking the temperature of a furnace, it is advisable to insert a
fireclay tube, closed at one end, in the furnace, the opening
facing towards the pyrometer, which is arranged to form an
image of the closed end of the tube. This gives very closely
the true temperature of the furnacd

PRACTICAL PYROMETRY 123

The radiation pyrometer is useless for measuring tempera-
tures if flames or heated vapours having high absorptive power
intervene between it and the object, and the same objection
appUes, although not quite in the same degree, to the various
forms of optical pyrometer. The advantage of these pyro-
meters in dealing with very high temperatures is that the
radiation increases very rapidly with the temperaturej namely,
in proportion to the fourth power of the latter, and very small
differences of temperature are therefore measurable. Further,
the radiation equations used in their calibration are in close
agreement with the gas scale throughout the whole observed
range, and extrapolation may therefore be resorted to with
confidence.'

The Cooling Curve

The simplest form of cooling curve is that illustrated in
Chapters II. and III., in which the temperature of the mass is
directly plotted against the time.^ In its construction, the
readings of the galvanometer are taken every 10 or 15 sees.,
and utilized directly for the construction of the curve. This
method is adapted to the measurement of freezing-points, but
not to that of changes taking place in the solidified mass, on
account of its comparative insensitiveness. Curves of this
type are referred to as " direct " cooling curves. Whilst they
indicate, with an accuracy dependent on that of the galvano-
meter, the temperature at which each change of state takes
place, and the range over which each development of heat
continues, they only give a rough idea of the magnitude of the
development, and a quantitative interpretation of the curve is
impossible. This is due to the fact that the rate of cooling is
not uniform, being different at the beginning and at the end of
the change of state. Direct cooling curves may, however,
be so modified as to yield quantitative information, by the

' On optical and radiation pyrometry, see L. Holborn, Brit. Assoc,
Rep., 1907, Leicester, 440; C. F^ry, ibid., 442; C. W. Waidner and G,
K. Burgess, Bull. Bureau Standards, 1904, 1, 189.

2 Frankenheim, Ann. Physik., 1836, [ii.] 39, 376 ; F. S. Schaffgotscii,
ibid., 1857, 102, 293.

124 METALLOGRAPHY

employment of an electric furnace, the temperature of which is
diminished at a uniform rate by the gradual increase of resist-
ance in the external circuit. The means of effecting this have
been described on p. 102, and the nature of the curves obtained
may now be discussed.

A perfectly inert body cooled in this way at a sufficiently
slow rate, will at each moment have the same temperature as
the furnace, and the readings of a thermo-couple in contact
with it, plotted as a temperature-time diagram, will give a
straight line {af'm Fig. 50). If a molten metal is cooling at a
sufficiently slow rate, the difference in the specific heats of the
liquid and solid will be without influence on the form of the
cooling curve, as the temperature of the metal will in both
cases be that of the furnace, ab and ef will therefore remain in
the same straight line. If freezing begins at b, the heat liberated
during crystallization causes the cooling curve of the metal to
assume the form bcde. The portion of the curve be is not quite
horizontal, but falls somewhat, the deviation being greatest
with substances of low thermal conductivity. The portion de
is also rounded off slightly. The area included between bcde
and the straight line be is proportional to the heat hberated
during freezing.^ This area may be measured, either by cutting
out in paper and weighing, comparing the weight with that of
a measured rectangle, as recommended by Plato, or more con-
veniently and accurately by means of the planimeter.

The measurement of this area provides a means of deter-
mining the latent heat of fusion^ a quantity which is required
to be known Avhen discussing the interpretation of the freezing-
point curve. It is necessary to know the latent heat of one
substance, and a comparison of the cooling curves of this
Substance and of the one under investigation, other conditions
being the same, enables the latent heat of the latter to be cal-
culated. For the method of calculation, reference must be
made to the original papers. The method has only been
applied hitherto to salts, and the latent heats of fusion deter-
mined by its means are in good agreement with those measured

' W. Plato, Zeitsch. physikal. Chem., igo6, 55, 721 ; 1907, 58, 350 }
I908, 63, 447.

PRACTICAL PYROMETRV 125

calorimetrically ; it is therefore to be expected that it will
prove of considerable value in the study of metallic alloys.
In Fig. 51, the types of curves obtained in Plato's furnace from
a mixture depositing crystals of a solid solution {a), and of an
alloy depositing crystals of a pure component, followed by
solidification of the eutectic {b), are shown. The exact form
of the arrests is clearly more easily distinguished than in
curves obtained in a furnace which is cooling freely by
radiation.

When the change under observation is not the freezing of
a liquid, but the transformation of one solid phase into another,

Fig. 50. — Cooling curve. Fig. 51.

the liberation of heat is generally neither so great nor so in-
stantaneous, and the indications on the time-temperature curve
become uncertain and difficult to interpret. They are brought
out more clearly by the device of plotting "inverse-rate" curves,
in which the temperatures are taken as ordinates, and the in-
verse rates of cooling (times taken for the temperature to fall
through a definite small amount) as abscissae. Denoting
temperature by 6 and time by /, the " direct-rate " curves have
as co-ordinates B and t, whilst the " inverse-rate " curves have
as co-ordinates B and dt/dO} A development of heat appears
on such a curve as a " peak," the position of which can be
located without difficulty, but on account of the uncertainty as

' The different types of cooling curves are discussed and compared by
G. K. Burgess, Bull. Bureau Standards, 1908, 5, 199 ; and W. Rosenhain,
Froc. Phys. Soc, 1908, 21, 180.

126 METALLOGRAPHY

to the points at which the curve leaves the neutral line and
returns to it, a quantitative interpretation is not feasible. The
" inverse-rate " method is of interest as having been employed
by Osmond in his original work on the allotropy of iron and
its alloys with carbon.^ For accurate work, a chronograph is
required to mark seconds on a tape, a key being depressed
each time that the E.M.F. increases by a fixed amount. In this
way, the time taken by the specimen to cool through i° or a
smaller interval is recorded.^

The rate of cooling, ddjdt, may also be plotted against the
temperature, instead of dtjdd. An apparatus, involving the use
of two thermo-couples, has been devised for this purpose.'

In accurate work on the transformations of solids, however,
one or other of the difference methods is most often adopted,
the temperature of the body under investigation being com-
pared with that of a body cooling without undergoing
polymorphic change of any kind. The curve representing the
development of heat in the transformation of a cooling metal
or alloy has then a very clearly marked character.

The irregularities occurring in difference curves, due to
differences between the specific heats and radiating powers of
the specimen and the neutral body, are largely eliminated in
the method of graphical representation due to Rosenhain
{loc. cit.), in which the co-ordinates are 6 and d{6 — B^jdQ,
that is, the slope of the difference curve is taken, and is plotted
against the temperature. Such a curve is called a "derived
differential" curve, since the quantity plotted against the
temperature is the differential coefiScient oi B — B^ with respect
to temperature, that is, it is the slope of the ordinary difference
curve.

The effect of the several methods of plotting is compared
in Fig. 52. A method of observation similar to that repre-
sented in Fig. 48 being used, the following readings of time,

' F. Osmond, Mim. A'rtill. Marine, 1887, IS, 573; J. Iron Steel
Inst., l8go, i. 38 ; Compt. rend., 1890, 110, 242, 346. The chronographic
method of obtaining inverse-iate curves is preferred to the difference
method by J. O. Arnold, Iniernat. Zeitsch. Metallographie, igil, 1, 192.

' F. Wiist, Metallurgie, 1906, 8, i.

' P. Dejean, Rev. de MHallurgie, 1905, 2, 701 ; 1906, 3, 149.

PRACTICAL PY ROME TRY

127

temperature, and difference of temperature between the two
bodies are obtained; each unit in t signifying 15 seconds.

aso

930

020

eio

i

^t

e-d.

" A ($-6.)

■A$

Ad

erature-time

Inverse rate

Difference

Derived differen

curve.

curve.

curve.

tial curve.

Fig. 52.

t

fl-fl,

/

e

9-9,

5

850-0°

8-5°

18

829-0°

II -0°

6

848-0

8-S

19

825-0

8-2

7

844-7

7-5

19-5

823-3

73

8

842-0

7-0

20

822-2

6-7

9

839-5

6-3

21

821-7

7-7

10

838-5

7-0

22

821-5

8-5

II

838-2

8-8

23

821-3

9-8

12

838-1

10-2

24

821 -I

lo-i

"3

838-0

12-0

24-s

819-0

9-5

14 .

837-9

13-6

25

815-0

6-0

15

837-5

'5-5

26

813-0

5-0

i5

8360

14-6

27

811-6

4-7

17

833-0

13-0

In curve A (Fig. 52), the vahies of Q are plotted, directly
against those of t, giving an ordinary cooling curve. Two
developments of heat are indicated, the second one beginning
. immediately at the close of the first. The curve B is the cool-
ing curve of a neutral body added for comparison. By reading
oif from the curve A the values of t at each successive 1° C,
and tabulating the differences, we obtain a further series of

128 METALLOGRAPHY

numbers, and the dififerences between successive values then
represent the time taken to cool through i°, which may be

represented by — j. These values may be obtained directly if

a chronograph is used. Plotting these differences against 0, we
obtain the curve C. The difference curve D, constructed by
plotting the values of {6 — O^) in the table against 6, shows the
developments of heat with great clearness, indicating both the
commencement and the termination. Lastly, the curve E is
obtained by measuring the change of {6 — 6) for each i° change
of 0, or, more strictly, the slope of D at each point, and plotting
these values against 6.

The close resemblance between the curves C and E will be
noticed. In both cases the area of a " peak " is proportional
to the development of heat due to the change in question, but
this fact loses much of its value since the area is very difficult
to determine. It will be seen that, whilst the commencement
of the change is indicated with great sharpness by both of these
curves, the end of the change is hardly to be detected^ and the
measurement of the area is consequently difficult to perform.

A method of plotting, giving very well-marked arrests,
suitable for the measurement of arrest times, has been employed
in the study of the ternary alloys of mercury.^ Pure mercury,
which remains liquid within the range covered by the experi-
ment, is cooled under the same conditions as the alloy under
investigation. Readings are taken of the time at which the
mercury and the alloy respectively reach certain temperatures,
that is, time readings are taken every 5° or 2°. This necessitates
a very slow rate of cooling, and the co-operation of two
observers. The two sets of readings, plotted separately, give
two curves, of which the mercury curve is smooth, whilst the
alloy curve presents the aspect of an ordinary temperature-
time curve with an arrest. The time readings of the mercury
curve are now subtracted from those of the alloy curve at the
same temperatures, and the differences are plotted against
temperature. In this derived curve, the ordinates are 6, and
the abscissae {t — t^/i^O. Examples of the ordinary curve,

' E. Janecke, Zeitsch. physikal. Chem., 1906, 57, £07 ^ 1907, 60, 399.

PRACTICAL PYROMETRY

129

and of the derived curve corresponding with it, are shown in
Fig. 53. The arrest-time is very easily measured 011 the latter.
The dip in the curves at the commencement of freezing is due
to undercooling, as will be explained in Chapter X.

In concluding this chapter, it may be well to add a few
general remarks on the preparation of alloys for thermal
examination. In the first place, it is of the highest importance
that the metals selected as the ingredients of the alloys should
be of the greatest attainable purity. Whilst one or two per
cent, of a foreign metal may be without marked effect on the

Time

Fig. 53. — Janecke's method of plotting cooling curves.

general form of the freezing-point curve, if duly allowed for in
plotting the results, this does not justify the use of material
containing such a proportion of impurity, since an element of
uncertainty is thereby introduced, and the effect on other
properties of the alloys may be much more considerable.
Polymorphic changes are sometimes affected to a marked
extent by the presence of small quantities of another element,
especially of a non-metal. Moreover, the determinations of
any physical properties of the alloys are rendered valueless
by such a procedure. Chemical and metallurgical literature
is already unfortunately overburdened with data respecting
physical properties of alloys, the material for which was in
too many cases taken from a laboratory stock without any
previous chemical investigation. In the writer's experience,
elaborate physical researches, involving prolonged and laborious
T.r.C. K

I30 METALLOGRAPHY

determinations of a high degree of accuracy, have been con-
ducted with material described as "copper wire," "zinc in
sticks," etc., which a chemical analysis would have shown to
be worthless for the purpose.

It is not difficult to procure metals of the requisite degree
of purity, except in a limited and diminishing number of cases.
Putting aside fine gold and silver, the high purity of which is
well known, the demands of the electrician and the engineer
have led to the production on the large scale of copper which
approaches the best scientific preparations in its freedom from
all but the minutest traces of impurity. The best qualities of
zinc, tin, antimony, lead, and aluminium are also of a very
satisfactory degree of purity. Iron, either in the form of the
best Swedish wrought iron or of the practically carbonless
steel used for transformer cores, is readily obtainable, and is
more satisfactory than electrolytic iron prepared in the labora-
tory, which is liable to contain impurities derived from the
electrolyte. In fact, it may be said that laboratory preparations
of metals, unless means of working on a fairly large scale
are available, are generally much less satisfactory than the best
commercial products. The less common metals may often be
obtained in a pure form from Kahlbaum. It is desirable that
all raw materials should be analysed before use.

It is a common practice to assume the composition of the
alloy from the weights of the components taken in its pre-
paration. Should one of these be volatile, as zinc or arsenic,
or readily removed by oxidation during melting, as manganese
or magnesium, an analysis is made of two or three of the
alloys after melting, and the loss computed and applied to
the correction of the other alloys of the series. This practice
is to be condemned. Variations in the amount of volatilization
or oxidation are very liable to occur, and it should be an
invariable rule that all alloys should be analysed after their
cooling curves have been taken. A complete analysis is not
of course necessary in every case, as if one metal in a binary
alloy be estimated accurately, the quantity of the other may
commonly be determined by difference. A few alloys of each
series should, however, be analysed completely, in order to

PRACTICAL PYROMETRY 131

detect the absorption of impurities during melting. This is
especially necessary when dealing with alloys of high melting-
point, which are very liable to take up silicon, etc., from the
crucibles in which they are contained or from the tubes used
to protect the thermo couples.

Before analysing an alloy which has been melted, the mass
removed from the crucible should, if possible, be sawn through
vertically. Any lack of uniformity in composition may generally
be detected in this way. Should the alloy have separated into
two liquid phases, these will be recognized as two layers in the
solidified mixture, or if stirring has been so vigorous that
separation has not taken place so completely, the one consti-
tuent may be seen distributed in the form of globules through
the mass, as in rapidly cooled alloys of copper and lead. When
the crystals which separate at the initial freezing-point are of
lower specific gravity than the mass of the alloy, they float up
to the surface, and cause a corresponding concentration of that
constituent in the upper part of the ingot. Thus, in ternary
alloys of lead, tin, and antimony, the cubical crystals approxi-
mating to the composition SbSn float to the surface, and are
seen on sawing through the ingot and filing to a flat surface,
without the necessity of polishing or examining under the
microscope. Should such a lack of homogeneity be recognized,
it is advisable to re-melt the alloy, and to cool rapidly with
constant stirring. It is better not to trust to making analysis
of the upper and lower portions of the ingot separately and
averaging the results.

The presence of non-metals as well as metals should be
looked for in the analysis if opportunity for their access to the
alloy has occurred. It seems probable that abnormal results
obtained by some investigators with alloys of manganese are
due to the presence of carbon (see p. 52, footnote), whilst the
same element is no doubt responsible for the fact that whilst
copper and iron are now known to alloy with one another in
all proportions,* they have been described as being immiscible

• J. E. Stead, J. Iron Steel Inst., 1901, ii. 104 ; H. Wedding and W.
Miiller, Stahlu. Eisen, 1906, 26, l\a,^ ; R. Sahmen, Zeiisch. a?iorg. CAem.,
1908, 67, I.

132 METALLOGRAPHY

in the liquid state.' It has in fact been shown by Stead that
iron containing carbon is not miscible with copper.

Lastly, reference should be made to the quantity of alloy
necessary for a satisfactory pyrometric investigation. Most
observers have employed very small quantities, of from lo to
20 grams, but there are serious objections to such a procedure.
Not only is it difficult to attain a sufficiently slow rate of cooling,
on account of the large surface presented by the alloy in pro-
portion to its weight, but the mass of the thermocouple and
its protecting tubes bears too large a proportion to that of the
alloy, and small changes in the direction of the cooling
curve may be entirely overlooked. The National Physical
I,aboratory, which has set a good example of careful work in
its published investigations, recommends from 120 to 300
grams as a suitable quantity of alloy for determination of
freezing-points. Similarly, Day and Sosman {loc. cit.) recom-
mend the use of a mass of metal measuring about 25 mm. in
diameter, and 45 mm. deep. There are, of course, several
difficulties, apart from that of cost of material, involved in
work on so large a scale, which may deter the investigator
who has not the resources of a large laboratory at his disposal.
The large crucibles required are both more costly and more
fragile than those which suffice in dealing with srnaller
quantities, and care is required in the selection of fire-resistant
materials for their construction if high temperatures are to be
employed. Larger furnaces are also necessary for heating the
alloys. The initial cost of the metals may be minimized by
using the same material for several alloys, removing portions
of the ingot after cooling for analysis and microscopical
examination and re-melting the remainder with a further
quantity of one of the components. Should the alloy be one
which acts chemically on the crucible, or absorbs gases from
the atmosphere of the furnace, such repeated re-melting has
the disadvantage of causing an accumulation of impurities,
which must be looked for on analysis.

Such precautions may appear excessive, but it is important

' J. Riley, J. Iron Steel Inst., 1890, i. 123 ; V. O. Pfeiffer, Metallurgie,
igo6, 3, 281.

PRACTICAL PYROMETRY 133

to insist on their necessity. Determinations of freezing-point
curves, if undertaken at all, should be performed with the
utmost available accuracy. The knowledge of alloys has now
reached a point at which there is no need for more rough
preliminary surveys, especially as the points which are of
the greatest theoretical importance are precisely those which
are most likely to be overlooked in an incomplete investigation.
On the other hand, an extensive field for research is afforded
by the transformations of soUd alloys, which are best studied
by the method of difference curves, and for these smaller
quantities, from 20 grams upwards, suffice, owing to the
superior sensitiveness of the method. Metallography^ more
than most branches of physical chemistry, has suffered in
the past from the accumulation of inaccurate data, and it is
well that the investigator should take every precaution to avoid
adding to their number.

CHAPTER VII

THE PREPARATION OF MICRO-SECTIONS

The preparation of thin, transparent sections, such as are
employed in the study of rocks and minerals, is not possible
in the case of metals and alloys, which remain opaque in the
thinnest slices. It is therefore necessary to examine prepared
surfaces by means of reflected light, only a single surface
of each specimen being as a rule utilized. Whenever the
metal is sufficiently soft to allow of it, the specimen is sawn
to the required size with a hack-saw. With a little practice, it
is not difficult to cut sections of regular form, with parallel
faces, from a piece of metal held in a vice, by means of a
hand-saw. Brasses, bronzes, etc., are more easily sawn than
such tough metals as copper or aluminium, in cutting which
some practice is required to overcome the drag on the teeth
of the saw. Soap solution should be applied to lubricate the
saw and to prevent heating, unless the alloy is of such a com-
position as to be affected chemically by water or alkali, in
which case turpentine may be employed. Electrolytic calcium
may be cut by moistening the saw with alcohol.

A convenient size for micro-sections is from lo to 15 mm.
square, and from 4 to 7 mm. in thickness. Larger specimens
are not easy to polish uniformly, but it may of course be
necessary sometimes to use larger sections when it is desired
to include the whole of some structure or flaw. Specimens
which are too thick are apt to rock slightly when held in the
hand during grinding and polishing, so that the surfaces
become rounded instead of flat. If too thin, the sections are
liable to " buckle," so that the centre becomes hollowed, and

134

THE PREPARATION OF MICRO-SECTIONS 135

escapes polishing. When thin sheet metal is examined, it
should be cemented to a piece of thicker metal to serve as a
support. Cross-sections of wires may be examined either by
drilling a hole in a small block of soft steel, into which the
wire may be fitted tightly by tapping with a hammer, or they
may be embedded in white metal, as described below (p. 145)
for the edges of sections.

When a number of sections have to be cut, or when the
ingots or bars to be sampled are of considerable size, a
mechanical saw is of great assistance. A hack-saw moved
horizontally in guides by means of a crank gives the best
results, the metal to be cut being clamped in a vice, and a
steady load, which may be varied according to the hardness
of the metal, being applied by means of a sliding weight.
Light saws of this kind, adapted to be driven by either hand
or power, are to be found in most metallurgical laboratories.
An automatic trip arrangement throws the saw out of gear
when the specimen is cut through. Soap solution should be
applied to the saw from time to time with a brush.

Care must be taken to ensure that the micro-section is
representative of the metal under examination. When there
is any doubt as to the homogeneity of the mass, sections
should be cut from both the outer and inner portions. Much
useful information is often obtained by cutting serial sections,
representing the change of structure on proceeding from the
surface to the centre of an ingot or of a forging. Ingots which
have been cast in iron moulds have a chilled outer layer which
may be quite different in structure and even in composition
from the general mass. The outer skin should also be
removed from small specimens which have been annealed or
quenched in the laboratory, as surface changes are likely to
occur during heating, even when care has been taken to avoid
the access of air, copper-zinc alloys losing a portion of their
zinc, stet Is becoming superficially decarbunzed, etc.

Many alloys are too hard to be cut with a saw, or so brittle
as to crumble under the pressure. It is then necessary to
break off a piece of the alloy with a hammer, and to grind a
flat surface. Hard alloys which are not brittle, such as certain

136 METALLOGRAPHY

steels, must be cut with a carborundum disc. For very hard
alloys, such as white cast-iron, or the alloys of iron with
chromium, tungsten, etc., an emery or carborundum wheel must
be used for grinding, care being taken that the supply of water
is sufficient to prevent heating. For alloys which are friable
without being so hard as white cast-iron, a more satisfactory
plan is to prepare a flat surface by grinding by hand on a
carborundum block, moistened with water or turpentine. A
torn or pitted surface is less likely to be produced in this way
than when a mechanically driven wheel is used. The specimen
should not in any case be allowed to heat up by friction,
■^hen a piece of metal is felt by the fingers to be distinctly
hot, the surface which is being rubbed is at a considerably
higher temperature, and in some cases, especially in that of
steels hardened by quenching, important structural changes
may be produced.

If the alloy is not too brittle, the sawn surface may now
be made approximately flat by means of a file. For this
purpose, a flat file is held in the left hand, with the end resting
on the bench, and the specimen, held by the thumb and finger
of the right hand, is drawn lightly up and down its surface.
The proper pressure to be applied . varies with the nature of
the metal or alloy, and can only be determined by experience.
It is to be remembered that the teeth of the file strain the
metal, especially if soft, to some depth below the surface, and
the lines of strain may reappear after polishing. It is often
better to avoid the use of a file, and to prepare the surface of
the specimen by rubbing with a circular motion on a piece of
coarse emery cloth laid on the bench. All sharp edges and
corners must be bevelled with the file before proceeding
further, in order to prevent tearing of the cloth or paper used
in the subsequent processes.

Preparation of a Smooth Surface

In the majority of cases, the preparation of a surface
suitable for polishing is effected by rubbing on emery paper.
Fine carborundum hones, such as are used for sharpening

THE PREPARATION OF MICRO-SECTIONS 137

knives, are also used, but are expensive, and are also liable to
become clogged when soft metals are being treated. French
emery papers, prepared for jewellers' use, are generally
employed, and can be obtained in four grades, o, 00, 000, and
0000, the last being the finest.^ The rubbing can now be per-
formed by hand or mechanically. Although the hand method
is naturally more tedious than the mechanical, it is to be
recommended to the beginner, and is in certain cases indis-
pensable. In order to carry it out, strips of the emery papers,
conveniently about 20 cm. long and 6 cm. wide, are placed on
a hard support, such as plate glass or hard wood. A con-
venient plan is to cement each strip to the surface of a small
school slate with seccotine. Before using a new paper, the
surface should be well rubbed with a piece of steel (an old
micro -section) to remove any coarse particles. There is no
advantage in preparing one's own emery papers.

The specimen, held between the fingers, is now rubbed
backwards and forwards on the strip of paper o with a light
pressure, care being taken to avoid tilting on reversing the
direction, which would cause rounding of the edges. The
rubbing is continued until, on examination with a hand lens,
no scratches are seen except the parallel series due to the
o paper. The process is now continued with the finer papers.
On passing from one paper to another, the specimen is turned
through a right angle, so that the new scratches cross the old
at right angles. In this way it is easy to see when the coarser
scratches have been effaced. On leaving the last emery paper,
the surface should reflect the image of a lamp or window
brilliantly, and should only show very fine parallel scratches
under a lens.

Much time is saved when hard metals are being examined
by employing a machine for polishing, by means of which the
emeity papers, attached to discs, are rotated rapidly while the
specimen is moved slowly across them. A horizontal disc,
mounted on a vertical spindle passing through a bearing and
resting on a footstep bearing, may be used, the papers being

' A still finer quality, marked 05, is now supplied by Messrs.
Dujardin & Co., Diisseldorf.

'38

ME TALLO GRA PHY

glued to detachable hard wood or brass discs. Wheels rotat-
ing in a vertical plane are used on the Continent and in
America, but are less convenient. The most useful form
of machine is that devised by Stead. In this form the

Fig. 54. — Polishing machine with five discs and automatic holders.

brass discs are attached to vertical spindles and driven from
below. The machine illustrated (Fig. 54) has five such
spindles, but smaller machines have only a single spindle.
Conical mahogany blocks, 6 cm. in diameter, fit on to the
brass discs and are held by friction, circular pieces of emery

THE PREPARATION OF MICRO-SECTIONS 139

paper being placed on these blocks and held in position by a
brass ring slipped over them. The discs being run at a high
speed (1200 revolutions a minute) by an electric motor, the
specimen is held in the fingers and moved slowly in the
opposite "direction to that in which the emery paper is travel-
ling. The scratches obtained in this way are very regular,
and the preparation of a surface fit for polishing is complete
with most metals in a few minutes. Hard steels, and other
metals which require long rubbing to efface the scratches, are
preferably held in a clamp. The machine shown is provided
with three such clamps. The specimen, held in a small vice
chuck or cemented to a flat chuck with plaster or glue, is
pressed on to the block by the holder seen in the figure, the
pressure being regulated by the spring and milled nut. This
holder is given a slow to-and-fro motion by means of the arm
shown, attached to an eccentric, so that the specimen is slowly
moved about on the rotating disc. The water vessel shown is
required for the subsequent stage of polishing. Machine
grinding should, however, only be employed with hard metals
or when a large number of moderately hard samples have to
be examined. For all delicate work, hand grinding is greatly
preferable, and is indispensable when soft alloys are under
investigation. It is impossible to develop satisfactorily by
mechanical grinding a eutectic containing lead.

Polishing

The process of rubbing on emery paper is really one of
cutting. Microscopical examination shows that the particles
of emery cut grooves in the metal, the section of each groove
being approximately parabolic. A certain amount of plastic
deformation also occurs in the neighbourhood of the groove,
this being the greater the coarser the scratch. Hence the
final scratches should be very fine in order to avoid the
presence of a deep surface layer of strained metal. The effect
of the emery on brittle metals, however, is to break out small
conchoidal chips, as in grinding glass, the chips being smaller
the finer the emery. It was long supposed that the effect of

140 METALLOGRAPHY

polishing with powders on cloth was merely to continue this
process, the grooves or pits becoming so fine as not to be
visible under the microscope. The recent researches of Lord
Rayleigh,^ Osmond and his collaborators,^ and Beilby,^ have
shown, however, that the process of polishing is essentially
different from that of grinding. The full bearing of these
investigations will be discussed in Chapter XVI., but here it
will be sufficient to say that the production of a polished
surface consists in the formation of a thin layer of " flowed "
metal on the surface, the irregularities of the scratched surface
being partly rubbed away and partly filled up. This plastic
behaviour of the surface layer is readily observable even in
such a brittle metal as antimony.

The powders employed in polishing are the oxides of alu-
minium, iron, magnesium, and chromium. French chalk and
other substances which have been used give very inferior results.
The most generally useful powder is alumina, prepared by
igniting ammonium alum and levigating. Rouge (a fine form
of ferric oxide) has been more used by metallographists than
any other oxide, and it undoubtedly produces the most brilliant
polish; but it has the disadvantage of causing an excessive
flow of the surface layer, so that on developing the structure
by etching, the outlines of the constituents are often found to
be blurred and confused. It is useful in polishing hard steels,
and, if very fine, in giving the finishing touches to metals of
moderate hardness. It should not be used for very soft
metals.

For most purposes it is not necessary to prepare the
powder in the laboratory. The Swiss product sold for jewellers'
use as " Diamantine No. 2 " gives excellent results, except for
the most dehcate work with high powers, and does not drag
the surface as does rouge. It is only when extremely fine
detail is to be developed that a finer powder is required, and

^ Proc. Roy. Inst., igoi, 16, 563.

^ F. Osmond and G. Cartaud, Rev. gin. dcs Sciences, 1905, 16, 51.
A summary of work on the subject.

= Proc. Roy. Soc, 1903, 72, 227 ; Electrochem. and Metall, 1903, 3,
806.

THE PREPARATION OF MICRO-SECTIONS 141

this should be prepared by levigation, as described by Le
Chatelier." Alumina, prepared by the ignition of ammonium
alum, is ground with water in a mortar, or, better, agitated
with water in a porcelain ball-mill in order to break up lumps.
It is then washed several times with N/iooo nitric acid to
remove soluble matter, being allowed to settle each time.
When it is found to settle with difficulty, the nitric acid is
replaced by distilled water, and the alumina is finally suspended
in water containing 1-2 c.c. of strong ammonia
per litre. The liquid with the powder in ? ?

suspension is now sucked up by means of a ~H

filter-pump into a pipette holding about a litre
(Fig. 55). The lower end of this pipette is
sloped steeply as shown, in order to prevent
adhesion of the deposit. The opening at the
lower end should be about 2 mm. in diameter.
The tap is now regulated so that a drop
falls about every 12 seconds. All that issues
in the first 15 minutes is rejected, since it con-
sists of gritty particles, which are useless even -^^^ ^,
after re-grinding. After this, the deposit formed Pipette for pre-
in the next three hours may be collected, but P^nng alu-

•' ' mina.

It IS still comparatively coarse and capable of
producing minute scratches ; it should therefore be used only
for hard steels and similar substances. The suspended matter
which issues in from 3 to 12 hours is fine, and is suitable for
polishing brass, bronze, and, in fact, the majority of ordinary
alloys. For the very finest work, alumina capable of remaining
in suspension longer than 12 hours may be used. The yield,
however, is exceedingly small, and in order to obtain such
material large quantities of the crude substance must be
treated. The simple apparatus described above is then
replaced by an elutriation apparatus, as used in the physical
examination of soils. ^

' Ren. de Mitallurgie, 1905, 3, 528.

' Messrs. P. F. Dujardin & Co., Diisseldorf, supply these powders
mixed with 3 to S parts of water, a. little dye being added to distinguish
the grades of fineness. Thus 3-hour alumina is coloured blue, 12-hour is
colourless, and 24-hour alumina pink.

142 ■ METALLOGRAPHY

For very soft specimens, such as the alloys of tin and lead,
the best results are obtained by polishing with chromium oxide,
levigated as described for alumina, only the very finest portions
being used.^ Fine magnesium oxide is very convenient, and
does not require any preparation.

The deposits obtained in the process of levigation should
not be dried, but should be stored in the form of a thin cream
with water in bottles with well-fitting rubber stoppers. In some
laboratories the suspended alumina is precipitated by the cautious
addition of dilute acid, and the precipitate mixed with shavings
of Castille soap and warmed on the water-bath, so as to form a
cream, which is then stored in the collapsible tin tubes used
for artists' colours. The addition of soap gives good results in
polishing steel, but produces undesirable effects in the case of
many alloys, and the author prefers to dispense with its use.

For polishing purposes, it is necessary to distribute the
powder on some support. Fine felt or cloth is generally used
for this purpose. The texture must be close and uniform, and
there must be no ribbed or raised pattern. Most samples of
cloth will be found to scratch soft metals, even after washing
thoroughly to remove gritty particles. The best quality is that
known in the trade as " beaver " cloth. Good results are also
obtained with a kind of velveteen sold for polishing purposes
under the name of " selvyt," but this wears out more rapidly
than a heavier cloth.

The fabric is tightly stretched over a hard surface, and for
hand polishing may be conveniently attached to a support of
glass or slate. The operation of polishing is, however, generally
performed mechanically, the specimen being lightly held in the
fingers against a rotating disc covered with fabric. On the
small machine illustrated in Fig, 54, the cloth is stretched over
a hard wooden cone, and clamped with a brass ring. A good
plan, when using diamantine, is to clamp two thicknesses of
selvyt on to the cone, diamantine powder being placed on the
lower cloth. When thoroughly wetted, the finer particles of
the powder work their way up through the fabric under the
pressure of the specimen. Usually a larger disc is employed ;

' W. Rosenhain and P. A. Tucker, Phil. Trans., 1908, 209a, 89.

THE PREPARATION OF MICRO-SECTIONS 143

)

one 1 5 cm. in diameter, mounted on a vertical axis and driven by
a leather or gut band by a treadle or motor, is very convenient
(Fig. 56). Thecloth is clamped
on by means of a brass ring.
In other forms of apparatus
the disc runs vertically, two
discs being mounted on the
same horizontal axis. The
disc should in either case be
surrounded by a metal water-
guard.

When dry powder is applied
to a cloth surface, it should be

Pig. 56. — Simple polishing disc.

thoroughly rubbed in with the finger, and a stream of water run
over the surface to remove coarse particles. It is much better
to apply the powder in the form of a fine suspension (contain-
ing 3 to 10 grams of solid per litre, according to the coarseness)
by means of a spraying bottle fitted with jet and rubber bulb.
The cloth is kept continually moist by the application of the
spray. Great care must always be taken to keep the polishing
cloth free from dust, and rubbing with the finger or a brush
and washing should never be omitted on resuming work after
an interval. A single particle of grit may score a specimen so
badly as to necessitate re-grinding and re-polishing.

All polishing is carried out on a wet cloth. This is essential
for microscopical work. The forced polish given by rouge on
dry chamois leather is very attractive to the eye, but is useless
for a surface which is to be etched. The machine shown in
Fig. 54 is provided with a central water vessel, from which a
small tap projects over each disc, by the regulation of which
water may be made to drip regularly on to the surface of the
cloth. The excess of water is thrown off by centrifugal force,
is caught by the brass water-guards, and flows into a trough
below the level of the discs. The water vessel is unnecessary
when the spray is employed. Alloys which are attacked by
water may be polished with a little light petroleum.

If the polishing powder cakes on the surface of the metal,
it is a sign that the cloth is too dry. If allowed to continue,

144 METALLOGRAPHY

" seizing " may take place, and it may be impossible to remove
the powder without destroying the surface. With many metals
a point is reached at which further polishing produces a dimi-
nution of the lustre, and the process should then be interrupted.
Examination from time to time with a hand lens enables the
operator to follow the gradual disappearance of the scratches.
When finished, the specimen should present a uniform mirror-
like surface under the microscope, unless either of the following
conditions is present : —

1. The constituents differ markedly in colour. In copper-
antimony alloys rich in antimony the structure is well seen on
polishing without etching, as the compound CuaSb, which is
present as one of the constituents of the eutectic, is violet in
colour, and outlines the antimony crystals with great distinct-
ness. The same may be said of the alloys of aluminium and
gold, the compound AuAlj (Rob arts- Austen's purple alloy)
being strongly coloured.

2. The constituents differ greatly in hardness. The hardest
constituent may be found to present itself in relief, even when
precautions have been taken to avoid wearing away the soft
parts. Thus in polishing white bearing metals containing
copper and antimony in a base of tin or of tin and lead, the
hard needles of the copper-tin constituent will almost always
be found to project above the soft matrix, and thus to be
visible without etching.

3. The alloy is porous. Dark irregular patches are seen.
Cavities in the specimen cause serious difficulties in polishing,
since they readily retain particles of dirt, emery, or polishing
material which may become dislodged in the subsequent pro-
cess of polishing, and give rise to deep scratches. A porous
specimen should therefore be thoroughly washed in a stream
of water after each stage. When the cavities are coarse, as
when defective castings or cracked specimens are under exami-
nation, it is advisable to fill them with a plastic material, such
as shellac, before rubbing on fine emery paper.

However carefully the polishing process is conducted, the
edges of the specimen will become slightly rounded. When,
therefore, the examination of the structure right up to the edge

THE PREPARATION OF MICRO-SECTIONS 145

is important, as in the study of fractures, some means of pre-
serving the edge must be adopted. The simplest plan is that
of embedding in white metal. The piece of metal to be ex-
amined is immersed in a crucible of molten white metal (tin-
lead-antimony alloy), and held in position by a wire until the
alloy has solidified. The mass is then sawn through in the
required direction, and the section thus obtained, in which
the metal to be studied is surrounded and protected by white
metal, is polished in the usual way.^

More delicate detail, such as the fine step-like deformations
of strained surfaces known as " slip-bands," may be protected
by electro-plating with copper.^ Steel specimens must first be
thinly coated by a weak current in a copper cyanide bath, a
thick plating then being applied in the usual acid sulphate bath,
several days being necessary.

Etching

In general, further treatment is required to render the
structure clearly visible. The means adopted are usually of a
chemical nature, and depend on the different degree of attack
of the separate micrographic constituents by reagents. It will
be convenient to begin with liquid etching reagents, and to
consider the various solutions in order.

Hydrochloric acid. — The concentrated acid is useful for
many white metals, the basis of which is tin. The specimen
is immersed in the acid for a few seconds, the process being
watched with a hand lens. When sufificiently etched, it is
removed, rinsed 'in water, lightly dried with a cloth, and
examined.

Dilute hydrochloric acid is a convenient reagent for metals
containing large amounts of tin, and also for steels in certain
cases. An alcoholic solution, prepared by diluting 5 c.c. of
concentrated acid to 100 c.c. with alcohol, is the most regular
in its action. The specimen may be washed with alcohol, and

' See A. K. Huntington, Trans. Faraday Soc, 1905, 1, 324.
"^ W. Rosenhain, Proc. Roy. Soc, 1905, 74, 557 ; J. Iron Steel Inst.,
1906, ii. 196,

T.P.C. L

146 METALLOGRAPHY'

dried in a current of air. Alloys containing free zinc require
a much more dilute acid.

Nitric acid. — The concentrated acid is used for metals
which can only be attacked by its means. A method of
etching formerly in favour for iron and steel was that of cover-
ing the specimen with concentrated nitric acid, which rendered
the surface passive, and rapidly rinsing in a stream of water.
The momentary action occasionally gave very good results,
but it was not under control, and has been superseded by
better methods. Nitric acid, D 1-2, is a more generally useful
reagent. It etches phosphor-copper, copper-silver alloys,
etc., very clearly, giving bold contrasts which are very
suitable for photography. It may also be used for the deep
etching of copper and its solid solutions. Dilute nitric acid
was formerly the most general reagent for steels, a i per cent,
or 5 per cent, solution being used. It gives good results in the
rough work of workshops, when fine detail is not required, but
is apt to cause much irregular "pitting." A 4 per cent, solu-
tion in amyl alcohol has been employed for steels, but has the
disadvantage that the coloured films produced by it vary in
tint in different parts of the same specimen. It serves to
distinguish austenite from martensite. Still better results were
found to be given ^ by a complex solution consisting of equal
parts of amyl alcohol, ethyl alcohol, methyl alcohol, and of
a 4 per cent, solution of nitric acid in acetic anhydride,
mixing just before use. The action of solutions in mixed
alcohols is very different from that of solutions in either
alcohol alone.

Picric acid. — This reagent, introduced by Ischewsky,^ is by
far the most generally useful for steel. A saturated alcoholic
solution is employed. The etching is very regular, without
pitting, and good photographic contrast is obtained. Since
the coloration produced by this and similar reagents is due to
the deposition of adherent films of carbonaceous matter, the
specimens should be washed with alcohol and dried by air
without wiping. Ether may be used to hasten drying. A

' W. J. Kurbatoff, Rev. de MHallurgie, 1905, 2, 169 ; 1906, 3, 648.
' Stahl u. Eisen, 1903, 23, 120.

THE PREPARATION OF MICRO-SECTIONS 147

4 per cent, solution of picric acid in amyl alcohol acts still
more slowly and uniformly.

Cupric ammonium chloride ' has been used for steels and
for copper, and an alcholic solution of iodine for many metals,
but neither solution is indispensable.

Ammonia gives good results with copper alloys, especially
with those of copper and zinc. Exposure to air is required,
so that instead of immersing the specimen, it is better to
apply the solution (i : 3) by rubbing lightly with the finger-tip
or a plug of cotton-wool. The development of structure is
easily followed. Immersion in ammonia to which hydrogen
peroxide has been added is less satisfactory.

Sodium or potassium hydroxide is employed for alloys
containing an excess of zinc or aluminium.

Sodium picrate, prepared by adding a cold saturated aqueous
solution of picric acid to a 50 per cent, solution of sodium
hydroxide, is the only reagent which blackens cementite
(FesC). The solution is kept boiling, and the specimen is
immersed in it for 5 minutes.

Ferric chloride. — This reagent, employed with great success
by Haycock and Neville in their study of the copper-tin
alloys,^ which includes some of the most beautiful photo-
micrographs yet produced, is generally useful for bronzes. It
is prepared by adding a syrupy solution of ferric chloride to
concentrated hydrochloric acid, and diluting with water, the
correct dilution having to be found by trial. Like ammonia,
it is apt to develop latent scratches, and a specimen presenting
an excellent polish may therefore appear badly scratched after
etching.

Aqua regia is necessarily employed for many alloys of
gold and of the platinum metals. It is generally necessary to
raise it to a temperature just below its boiling-point.

Of other complex solutions, a mixture of 3 vols, of a
saturated alcoholic solution of ^'-nitrophenol and i vol. of a

' E. Heyn, Verh. Ver. Bef. Gewerbejleisses, 1904, 235. A comparison
of the reagents for steel will be found in Goerens, Erstarrung und
Umwandlt/ngen von Eisenkohlenstoffiegierungen, Halle, 1907.

2 Phil. Trans., 1903, 302a, i.

148 METALLOGRAPHY

4 per cent, alcoholic solution of nitric acid has been found to
give excellent results with hardened steels.' Another reagent,
distinguishing clearly between the constituents of steels
quenched from a high temperature, is a 5 per cent, alcoholic
solution of w-nitrobenzoic acid," whilst a dilute solution of
sulphurous acid, which causes severe pitting in ferrite, has
been found useful in the examination of hardened steels.'

The reagents enumerated are the most important, but
many others find application for special purposes, and a con-
sideration of the chemical characteristics of the alloy under
examination will usually suggest the reagent to be employed.
Potassium cyanide is useful as a means of removing the
tarnished films produced in the etching of copper alloys.

Electrolytic etching. — It is often convenient to assist the
etching process by means of an electric current, the specimen
being made the anode in a solution of an electrolyte. This
may be ammonium nitrate or any other neutral salt. Le
Chatelier'' obtained the best results with copper-tin alloys
by using sodium thiosulphate. Ammonia may be used for
copper alloys. The solution is placed in a beaker, and the
specimen supported below the surface by a bridge of platinum
wire or foil, a second piece of platinum foil being employed
as cathode. The source of current is a dichromate cell or
small accumulator, a sufficient resistance in the form of wire
being interposed. Strong currents, up to i amp., have been
used in certain cases, but generally a very small current, o'oi
amp. or less, is sufficient. The process may be interrupted
from time to time in order to examine the etched surface.

Heat-tinting. — When heated in the air, many metals and

alloys become coated with a film of oxide, which remains

, adherent and shows the colours of thin films. Since different

constituents oxidize with unequal rapidity, Stead ° has based

' W. J. Kurbatoff, Rev. de MHallurgie, 1905, 2, 169 ; P. Breuil, Bull.
Soc. Ind. min., [iv.] 6; Melallurgie, 1908, 6, 59.
^ C. Benedicks, J. Iron Steel Inst., 1908, ii. 237.
' S. Hilpert and E. Colver-Glauert, J. Iron Steel Inst., 1910, ii. 54.
* Bull. Soc. d^ Encouragement, 1896, [v.] 1, 559 ; Etude des Alliages,(>l.
' J. Iron Steel Inst., 1900, ii. 137.

THE PREPARATION OF MICRO-SECTIONS 149

on this fact a very delicate method of distinguishing between
certain of the constituents of iron and steel. The specimen
is best embedded in sand on a metal plate or in a porcelain
capsule, and heated with a small flame until the colours appear.
The oxidation can be stopped at any required point by plung-
ing into mercury. To obtain successful results, the surface of
the specimen must be thoroughly clean and dry, as the presence
of moisture in cavities of the metal causes the formation of
irregular coloured rings. It is advisable to heat the specimen,
previously freed from grease, to 120°, and then to rub on clean,
warm flannel, before heating to a higher temperature. Very
regular heating is required, and as it is of great advantage to
keep the surface under observation throughout. Stead has
devised an electrical heater, by means of which the heat-
tinting can be carried on on the stage of the microscope
without injury to the latter. This device is shown in Fig. 57.
Two brass plates, provided with binding screws, are fixed
on an ebonite stage plate, with a circular opening to receive
the heating vessel. This is a porcelain capsule, partly filled
with a plastic magnesia |=^
mass, on which a flat spiral Capsule
of platinum wire is laid, and

'7^

fixed with a mixture of silica Fig. 57.

and sodium silicate, with which it is lightly covered. Two
brass strips form the terminals of the spiral, and serve to make
contact with the brass supports. The hemispherical capsule
can be freely tilted for the purpose of levelling the surface of
the micro-section, and the non-conducting nature of the
magnesia prevents the heating of any part of the microscope.
By suitably regulating the current, the heating is kept under
perfect control.

This method gives remarkably beautiful results with steel,
cast-iron, and certain copper alloys, especially those containing
antimony or silver. As an example of its use, the effect of
heating steel or iron containing phosphorus^ may be men-
tioned. At 280°, the carbide of iron, cementite, becomes
reddish-brown. The phosphide becomes pale yellow in 10

' Stead, loc. cit.

1 50 ME TALLOGRAPHY

minutes, salmon in 15, and on further heating assumes a
characteristic (heliotrope) shade of blue.

Coloured films may also be produced by the action of
iodine, bromine, or hydrogen sulphide on alloys, the dry
vapour being led, mixed with air, into a glass vessel containing
the specimen, warmed if necessary. Surface tinting has certain
advantages over etching, since none of the metal is removed,
and a flat surface remains, which is readily exposed again by
a momentary re-polishing. It is often useful to produce an
oxidation-film on a specimen, and after noting the appearance
or recording it by photography, to develop the structure by
etching, a comparison of the two observations giving results
of considerable value.

Polishing in relief. — By polishing with rouge on a soft
bed, such as chamois leather or parchment, the hard constitu-
ents of an alloy may be made to appear in relief. This
method is less employed now than in the early days of
metallography, when the choice of etching reagents was more
limited. A certain amount of sharpness of outline is inevitably
lost when a specimen is polished in relief, since the projecting
crystals are always to some extent rounded at the edges by the
friction. The method gives useful information in certain cases
as to the wearing quality of a metal intended for use in bear-
ings, etc., since it shows the quantity and distribution of the
hard particles in the soft ground-mass. For all other purposes,
the attack by reagents is to be preferred.

Polish-attack.— ^K method devised by Osmond ' consists in
combining the polishing process with one of selective colouring
with a reagent. The fact that many workmen use an aqueous
extract of liquorice root, known as coco, for colouring steel,
suggested the use of this preparation. A piece of wet parchment
is fastened to a board, and fine rouge or precipitated calcium
sulphate is rubbed into its surface, all but the finest particles
then being removed by washing. The specimen is then
polished very lightly, keeping the parchment moistened with
the liquorice extract. After a short time, the steel becomes
selectively attacked, and the structure is very clearly developed.

' r. Osmond, Atude des Alliages, 1901, 277,

THE PREPARATION OF MICRO-SECTIONS 151

The use of a vegetable extract of uncertain composition having
many disadvantages, Osmond made experiments with various
inorganic salts, and found that a 2 per cent, solution of am-
monium nitrate gave equally good results, and was more rapid
in its action. The extract of liquorice probably owes its
activity to the presence of similar salts.

This method has been especially successful in the hands
of its inventor, and the beautiful photo-micrographs of pearlite
(the finely laminated alternating growth of iron and iron carbide
found in steels, see Chapter XVII.) prepared by him, and
frequently reproduced by other writers, were obtained by its
means. For the ordinary examination of steels, it has been
practically superseded by the picric acid method. Le Cha-
telier^ has obtained good photographs of grey cast-iron by
rubbing the surface lightly with an alcoholic solution of
iodine. The beneficial effect of gentle friction in etching
copper-zinc alloys with ammonia is also very marked.

It has been proposed ^ to deposit a thin layer of metal on
the polished surface, the different electrochemical character of
the various micrographic constituents causing selective deposi-
tion. The section is either made the cathode in an electrolytic
cell, or is simply immersed in a solution of a metallic salt, the
local couples formed on the surface being then sufficient to
cause deposition. The method has been utilized to develop
the " cores " in solid solutions in a state of imperfect equili-
brium. Thus, on immersing a polished specimen of unannealed
tin bronze containing 16 per cent. Sn and 84 per cent. Cu in
a 10 per cent, solution of copper sulphate for 15 seconds, dark
cores are seen in the crystals of the soUd solution. The results
are, however, inferior to those obtained by heat-tinting or by
etching with a suitably chosen reagent.

Preparation of a Smooth Surface vcithout Grinding
OR Polishing

The great difficulty experienced in grinding soft metals and
alloys has led some investigators to devise other means of

* Etude des AUiages, 42 1 .

' F. Giolitti, Gazzeita, 1906, S6, ii. 142 ; 1908, 38, ii. 352,

152 METALLOGRAPHY

obtaining a smooth surface. In experiments on the deforma-
tion of lead, Rosenhain ^ prepared a clean surface of the
metal by scraping, and levelled this surface by pressing heavily
against plate glass. On subsequent annealing in a sealed glass
tube, a perfectly smooth surface was obtained, on which the
lines obtained by deformation could be accurately studied.
This method is, of course, only applicable when the metal is
to be annealed after the pressure.

Many investigators have used cast in place of cut and
polished surfaces. The examination of the upper surface of
a bead or ingot ^ or of the under side of a crust obtained by
pouring off the still liquid portion of an alloy * gives interesting
information as to the growth of a crystal ; but such an examina-
tion naturally supplements the study of polished sections rather
than replaces it. Alloys of comparatively low melting-point
may be cast on a surface of glass ^ or mica/ or a heated sheet
of one of these materials may be slid over the surface of a
small crucible filled with the molten alloy. The structure thus
revealed, although often very distinct, does not necessarily
correspond with that in the interior of the mass. Curious
effects are sometimes produced by the escape of gas at the
moment of solidification, causing the inclusion of bubbles
between the metal and the glass, these bubbles taking the
form of negative crystals.

The improvements which have been made in recent
years in the processes of grinding and polishing soft metals
have rendered these substitutes almost superfluous, except
when it is specially desired to study the cast surface.

The preservation of polished and etched specimens is a
matter of some difficulty. The usual method of preserving
microscopical preparations, by affixing a cover-glass with
Canada balsam, is not available, as the presence of the cover-
glass interferes with the subsequent examination under vertical

' y. Iron Steel Inst., 1904, i. 346.
^ W. Campbell, Metallurgie, 1907, 4, 801, 825.

' J. C. W. Humfrey, Phil: Trans., 1902, 200a, 225 ; N. S. Kurnakoff
ard N. A. Pushin, Zeitsch, anorg. Chem., 1907, 62, 430.

* J. A. Ewing and W. Rosenhain, Phil. T^ans., 1900, 193a, 353.
^ H. J. Hannover, Btill. Soc. d' Encouragement, 1900, [v.] 6, 210.

THE PREPARATION OF MICRO-SECTIONS 153

illumination, causing scattering of light and " flare." The sur-
face may be coated with a thin layer of a solution of nitro-
cellulose in amyl acetate. The film thus obtained is very thin,
and may be removed if necessary just before examination, by
washing with the solvent. The most generally satisfactory
plan is that of heating the specimen in an oven to near 100°,
and packing in a labelled glass tube containing warm, dry
cotton-wool. All specimens are most conveniently stored in
an unmounted condition. Heat-tinted sections retain their
appearance well.

Sections in an unpolished condition may be stored in the
small envelopes, about 3 cm. square, used by gardeners for
collecting seeds.

CHAPTER VIII

THE MICROSCOPICAL EXAMINATION OF PREPARED
SECTIONS

Although several special types of microscope, each possessing
some advantages, have been designed for the purpose of ex-
amining the structure of alloys, it is quite possible to obtain
satisfactory results with any microscope of good mechanical
construction. Since metals and alloys, being opaque, are
necessarily examined by reflected light, the substage, con-
densers, polarizing prisms, and other appliances which add
greatly to the cost of a good microscope, may be dispensed
with, whilst certain special illuminating devices are required
in their stead. It is nevertheless true that a microscope,
designed with a view to its employment in metallographic
work, offers far greater convenience, especially when a large
number of different sections have to be examined in rapid
succession. The beginner, however, who possesses a fairly
good ordinary microscope stand, need not be deterred by his
lack of the elaborate appliances described in the makers'
catalogues, and in the following account of the microscopical
equipment employed in metallography, his requirements will
be kept in view.

The principal modification found in metallographic micro-
scope stands is the provision of means for raising and lowering
the stage. The reason for this arrangement is, that when the
object is illuminated by means of a device inserted in the tube
of the microscope, as described below, it is convenient to fix
the position of this illuminator once for all, so that it always
occupies the same position relatively to the source of light.

•54

EXAMINATION OF PREPARED SECTIONS 155

When, owing to the use of an objective of different working
distance, or to the introduction of metal sections of diflFerent
thickness, it is required to vary the distance between the stage
and the front lens of the objective, it is convenient to effect
this by racking the stage up or down, leaving the body-tube,
in which the illuminator is placed, untouched. But even this
modification, although undoubtedly effecting a great saving of
time and trouble, is not indispensable for accurate work, and
means will be described below for dispensing with it.

Since the principal source of difference between metallo-
graphic and ordinary microscopical manipulation lies in the
illumination of the specimen by reflected, instead of by trans-
mitted light, it will be convenient to commence the description
of the appliances used with the illuminator.

In the simplest method, a beam of light is allowed to im-
pinge on the surface of the section at an oblique angle, as in
Fig. 58. With a truly plane surface, the whole of the beam is
reflected as by a mirror, and
passes entirely out of the field
of the objective. To an eye
placed at the end of the micro-
scope tube, therefore, the surface
of the metal appears perfectly
black, none of the reflected rays
being received by the eye. Should
the specimen be etched, however,
the surface will not be plane,
but will present a number of elevations or depressions, com-
posed of intersecting crystal faces. Suppose that some of
these faces, as at a, a, have such an inclination to the surface as
a whole that the rays which they reflect have a vertical direction.
These rays will enter the microscope, and the areas a, a will
consequently appear bright. In an etched portion of the
specimen, there will usually be numerous small facets having
the required inclination, and the etched area will therefore
appear to be irregularly illuminated, and to have a granular
appearance, owing to'its roughness. On the other hand, those
areas which have not been attacked by the etching reagent

Fig. 58.

156 METALLOGRAPHY

retain their original smoothness, and since they reflect the
incident beam outside of the field of the objective, they present
the appearance of dark areas on a light background. The
appearances presented by sections under this form of illumi-
nation are often difficult to interpret, and are apt to mislead
those who have not considerable metallographic experience.
For this and other reasons, amongst which the great loss of
light is the chief, oblique illumination is of only limited utility ;
it is, however, indispensable for certain kinds of investigation,
as in distinguishing the true nature of slip-bands, etc. When
it is to be employed, the beam of light is concentrated on the
object at a suitable angle by means of a bull's-eye condenser,
the position of the condenser being so chosen that the area of
illumination is as uniform as possible. The parabolic reflectors
formerly used for this class of work are only of limited use,
and need not be described. When grazing incidence is
required, an Amici prism (a small triangular prism, of which
two faces are convex) is convenient.

The majority of metallographic sections are best examined
under vertical, or nearly vertical, illumination. This necessi-
tates the introduction of a beam of light into the tube of the
microscope above the objective, this beam being then deflected
so as to pass through the objective, and after reflection from
the surface of the metal, to retrace its path, passing up the
body-tube of the microscope to the eyepiece end. Two different
types of illuminating device have been employed for this pur-
pose. In the first, known as the Beck vertical illuminator, a
thin plate of glass, with parallel surfaces, is placed at an angle
of 45° to the optical axis, as shown in Fig. 59. The rays from
the source of light, entering the tube in the direction AB, are
in part reflected from the surface of the glass plate, and pass
downwards in the direction BC. The rays reflected from the
object travel back in the direction CBD, passing through the
glass plate on the way. It will be seen that a considerable
loss of light is unavoidable on this system. Only a part of the
beam AB is reflected downwards, the remainder passing on in
the direction BE and being lost, and, in a similar manner, only
a part of the returning beam passes through the plate without

EXAMINATION OF PREPARED SECTIONS 157

Fig. 59.
to astigmatism are

change of direction, the remainder being reflected from the
face, and passing uselessly out of the tube towards the source
of light. In spite of these disadvantages,
the Beck illuminator is generally to be
preferred, as being less liable to produce
false or distorted images than any in-
strument in which, as described below,
only a part of the objective is illumi-
nated, It is not essential that the small
plane reflector should be optically worked
so that its two faces are truly parallel.
An ordinary thin microscopic cover-glass,
tested for flatness, gives perfectly satis-
factory results, and has the merit of
cheapness. If too thick, defects due
produced.

The Beck illuminator, as ordinarily constructed, has the
defect that the whole aperture of the objective is not utilized,
as the turning of the plate into a plane inclined to the axis at
45° causes its projection to become an ellipse. It would be
an advantage to use a reflector so large that when inclined the
whole of the back lens of the objective is covered by it. This
pattern has been employed in Swift's microscope, designed by
Jackson and Blount, and in an illuminator designed by John-
stone Stoney, and made by Watson. The arrangements for
tilting, and for admitting the illuminating beam obliquely, in
the latter instrument are, however, not advantageous, as false
effects are thereby liable to be produced in the image.

For use with very low powers, where the working distance
between lens and object is great, a large square cover-glass,
mounted on a hinge, may be placed below the objective, as
suggested by Stead. ^

In the second form of vertical illuminator, the thir; glass
plate is replaced by a small right-angled prism, giving total
reflection. Since the prism necessarily blocks out a portion of
the beam, it is necessary to arrange it so that only one half of
the area of the tube is thus occupied. The reflected ray, after
' J. Iron Steel Inst., 1897, i. 42.

158

METALLOGRAPHY

Fig. 6o.

passing twice through the objective, returns through the other
half of the tube, passing at the back of the prism, as shown in
Fig. 60. The prism is capable of a cer-
tain amount of rotation, by which the
inclination of the incident light may be
changed within certain limits. In the
Nachet form of illuminator, a traversing
movement is also provided, allowing the
prism to be moved nearer to or farther
from the centre of the microscope tube.
This adjustment is undesirable, displace-
ment in this direction causing consider-
able distortion of the image, and conse-
quent uncertainty as to the true meaning
of apparent structures observed. In the Zeiss form, diaphragms
of different shapes are provided for cutting off the marginal rays,
whilst the provision of an iris diaphragm between the illuminator
and the objective is common to many forms of apparatus
supplied by different makers. Although an apparent improve-
ment in definition is often obtained by such means, the in-
creased clearness is sometimes obtained at the cost of the
truth of the image. It is always better to cut down the
illuminating beam before entering the microscope, by means
of suitable diaphragms placed between the source of light and
the vertical illuminator.'

In one form of illuminator supplied with the Rosenhain
microscope made by Messrs. Beck, the reflector takes the form
of a silvered half-disc, inclined at an angle of 45°. The mode
of action of this apparatus exactly resembles that of the prism.
A further modification of the prism, by which the central
bundle of rays is utilized, is described below in connection
with the Reichert microscope.

Whatever be the construction of the vertical illuminator,
the prism or disc is enclosed in a short tube which screws into

' The great superiority of the plane plate illuminator over any form
which utilizes only a portion of the bundle of rays entering the apparatus
is shown by a comparison of the efficiency of the two forms in rendering
fine detail. C. Benedicks, Metallurgie, 1909, 6, 320.

EXAMINATION OF PREPARED SECTIONS 159

the lower end of the microscope body-tube, the objective,
again, screwing into it. The illuminator must be capable of
rotating freely about its axis without unscrewing. This move-
ment, together with the movement of rotation of the reflector
about the transverse axis, provides the means of adjustment
of the incident light. In order to avoid internal reflections,
which give rise to loss of light and to " flare," the distance
between the reflector and the back lens of the objective
should be as short as possible, nosepieces should therefore be
dispensed with, the slight extra trouble thereby necessitated in
changing objectives being more than compensated for by the
better optical results obtained.

The introduction of the vertical illuminator increases the
distance between the lower end of the body-tube and the stage
very considerably, and with the short limb possessed by many
stands, it will often be found, when working with low-power
objectives, especially with low-power apochromats, which have
very long mounts, that the tube has been racked up to its
furthest limit before focus is obtained. A microscope specially
constructed for metallography should therefore have a specially
long limb. With an ordinary small microscope, the difficulty
may sometimes be overcome by placing the specimen to be
examined beloiv the stage, attaching the glass slide by means
of rubber bands, and viewing the surface through the central
opening of the stage, thus securing an increased working
distance. If the microscope has a racking sub-stage, this may
very conveniently be employed as a stage with simple attach-
ments which will readily suggest themselves. By such devices
as these, satisfactory results may be obtained with an instru-
ment such as is to be found in any laboratory.

The next fitting to be considered is the stage. Since
metallographic specimens are only ground flat on a single
surface, they are often otherwise irregular in shape. In order
to examine them, it is necessary to set the flat polished surface
accurately perpendicularly to the optical axis. Most metal-
lurgical microscopes were formerly fitted with tilting or levelling
stages for this purpose, the specimen being mounted with
sealing-wax or cement on a glass slip, which is then tilted

i6o METALLOGRAPHY

mechanically until all parts of the surface remain in focus when
the slide is moved in any direction on the stage. Adjustment
by means of a levelling stage is, however, tedious, and is
rendered entirely unnecessary by the adoption of a simple device
due to Stead. This consists in mounting the specimen in such
a way that the surface to be examined is accurately parallel with
that of the glass slip. On placing the slip on the stage, and
holding it in the usual way by means of clips, this parallelism
is retained, and no further adjustment is needed. For this
method of mounting, the only apparatus required consists of a
number of rings, cut from a piece of brass tube about 3 cm.
diameter, of widths varying from 4 mm. to 16 mm., the edges
being accurately parallel. Such rings are readily cut on a

/Glass sli de

Brass rin^-

- - Pleisticin&
\ — Specimen

Giass plate-

Fig. 61. — Mounting device.

lathe. The micro-specimen is laid, with the smooth surface
downwards, on a sheet of glass, and a ring of suitable height
placed over it. A small quantity of plasticine (a plastic
material used for modelling purposes, which may be used
indefinitely without becoming hard) is placed on the specimen,
and an ordinary glass micro- slip is pressed down until it is in
perfect contact with the brass ring (Fig. 61). The slip, with
the specimen now adhering to it, is lifted, turned over, and
placed on the microscope stage, and requires no further
levelling. Instead of using a series of rings of varying thick-
ness, mechanical mounting devices have been used, on the
principle of the well-known botanical microtome,' or of the
Rousselet compressorium.^ The construction will be obvious
to any one acquainted with microscopical apparatus, but the
cheap and simple device described above will be found equally
satisfactory in all respects.

' J. E. Stead, J. Iron Steel Inst., 1897, i. 42.

* W. Rosenhain. Apparatus made by Messrs. Watson.

EXAMINATION OF PREPARED SECTIONS i6i

The provision of mechanical movements to the stage, and
of means for rotating it, greatly adds to the convenience of
the worker, and the former is in fact indispensable when
working with any hut low powers. Rotation is necessary
when observing structures under oblique illumination. The
mechanism does not differ from that employed in microscopes
intended for observations by transmitted light. A large,
roomy stage, free from projections, is convenient when a
variety of specimens have to be dealt with.

In laboratories in which a large number of specimens have
to be examined, especially if photo-micrographs are to be
taken, it is of advantage to use a stand which is designed with
a view to metallographic work. For photographic purposes,
the horizontal position is to be preferred to the vertical, on
account of the greater rigidity and freedom from vibration, as
well as for the increased convenience and comfort of the
observer. The first instrument of this kind was that devised
by Martens for use in the Charlottenburg laboratory.^ This
form is the parent of several later instruments, including that
of Messrs. Watson (Fig. 62). The focussing, as in all these
special forms of microscope, is effected by racking the stage and
not the body-tube. The fine adjustment, however, is in most
forms attached to the body-tube in the ordinary way. An
entirely different construction was adopted by Le Chatelier,^ in
whose instrument the specimen under examination is placed
face 1 downwards on the stage, the illuminating beam entering
through a horizontal coUimating tube, and being reflected
upwards by a totally reflecting prism into the objective, a
second prism directing the reflected beam into the tube con-
taining the eyepiece. The same principle has been adopted
in the new microscope by Messrs. Reichert,^ a diagrammatic
section of which is shown in Fig. 63. The defects of the

» A. Martens and E. Heyn, Mitth. k. iechn. Versuchs-Anst., 1899,
17, 73. This paper contains a full description of the Charlottenburg
equipment, and a valuable discussion of the best optical conditions.

"" H. Le Chatelier, Etude des Alliages, 1901, 421. This instrument is
made by Messrs. Pellin, Paris.

' See the description by O. Heimstadt, Mdallurgie, 1909, 6, 58.
T.P C. ^

l62

METALLOGRAPHY

ordinary illuminating prism are in this instrument partly
overcome by the use of two right-angled prisms cemented
together to form a cube, P. The upper prism is silvered
over an elliptical area, as shown by the central dark line.
The rays entering the instrument from the mirror M pass
through the glass cube in the form of a hollow cylinder of

Fig. 62. — Horizontal microscope for metallography.

circular section and so enter the objective. The surface of
the specimen S is thus illuminated by an oblique cone of
light, and the rays from a central bundle are reflected
by the silvered ellipse so as to pass into the observing
tube O. This arrangement, by providing for conical instead
of one-sided illumination, partly avoids the exaggeration
of the spherical aberration defects of the objective, noticed
when an ordinary vertical prism is used. It is, however.

EXAMINATION OF PREPARED SECTIONS 163

liablfe to the objection already urged against all devices in
which -only a portion of the illuminating beam is employed,
namely, that it is liable to give a false rendering of fine
detail.

The Rosenhain metallurgical microscope (Fig. 64) is so
constructed as to have great rigidity wlien used in a hori-
zontal position. The usual construction of the stand is de-
parted from widely, and the massive girder-shaped limb,
fitting into a shaped portion of the heavy triangular base,'
removes all risk of displacement by shaking. The stage is

Fig. 63. — Reichert's moclificadon of Le Chatelier's microscope.

fitted with both coarse and fine adjustments for focussing,
and there are numerous minor modifications for facilitating
illumination.

The objectives to be used in metallographic investigations
require careful selection. Since metallic specimens are
examined without a cover-glass, which would cause excessive
surface reflection and consequent loss of clearness, all
objectives should be corrected for uncovered objects. Further,
in order to avoid internal reflections,' the back lens of the
objective should be as close to the vertical ' illuminator as
possible, and it is therefore advisable to have special short

1 64

METALLOGRAPHY

mounts, which have the additional advantage of reducing the
height to which the body-tube has to be racked up. Several

Fig. 64. — Rosenhain microscope.

of the leading makers now supply such objectives, in short
mounts, and corrected for uncovered objects.

The most useful powers for metallographic work are 25,
16, 6, 4, and 2 mm., with a low power such as a 50 mm. for

EXAMINATION OF PREPARED SECTIONS 165

very coarse structures. To obtain the best results, apochro-
mats should be used. Good achromatic objectives are much
less expensive, and are quite satisfactory for ordinary routine
observations ; but when photographic records of fine detail
have to be made, the superior performance of the apochromats
will be found fully to justify their increased cost. Apochromats
do not, however, give such a flat field as achromatic lenses.
For high-power work, an oil immersion objective must be
used. The results obtained are apt to be disappointing at
first, but the lens is not entirely responsible for this, as it is
very difficult to prepare the specimens with a sufficiently good
surface to bear the high magnification. For work of the highest
class a lens of large aperture, such as Zeiss's 2 mm. apochromat
of ap. I "4, should be employed. It must be remembered that
such an objective as this requires careful handling, on account
of the very high curvature of the lens, which is only held in its
mount by a shallow groove. Messrs. Swift also construct a
lens suitable for the purpose. There is sometimes an advantage
in using immersion, even for a lower power, on account of the
superior light -gathering power and increased working distance.
A \ in. (4 mm.) immersion lens is made by Messrs. Beck for
this purpose.

Nosepieces should be avoided, as they increase the distance
between the illuminator and the back lens of the objective, and
so cause liability to flare. The improvement in the perform-
ance of the objectives more than compensates for the slight
additional time occupied in changing objectives. It may also
be necessary to insert a lining of thin velvet or black paper in
the lower part of the body-tube to lessen reflection.

Ordinary Huyghenian eyepieces are used with achromatic
objectives ; but the specially designed compensating eyepieces
should always be used with apochromats to utilize their
advantages fully. The eyepieces should be of low or medium
power, as the loss of light in metallographic examinations is so
great that a high-power eyepiece gives a very dimly illuminated
field. A number 2 eyepiece will be found the most convenient
for ordinary routine work. When the apparatus is arranged
for photography, an eyepiece may be dispensed with, although

1 66

METALLOGRAPHY

it will then be found difficult to avoid a slight central flare, so
that a projection eyepiece is almost indispensable. This is an
eyepiece of low power (the X 2 is the most generally useful)
in which the lower lens produces an image in the plane of the
diaphragm, which is then magnified by the upper combination.
In order to focus this image, the tube holding the upper
combination may be moved in and out, revolving in a helically
cut slot. The position is indicated on the cap by a pointer
travelling round a dial (Fig. 65). When
projecting on to a screen, this part of
the eyepiece tube is moved until the
image of the diaphragm appears perfectly
sharp on the screen. The position of the
lenses in the eyepiece depends on their
distance from the screen.

Either electric light or gas may be
used as an illuminant. Where the appa-
ratus is fitted up in a permanent manner,
it is advantageous to use the electric arc,
on account of the great intensity of the
light, which reduces the length of expo-
sure necessary in photographing sections,
and facilitates working with high magni-
fications or with surfaces of low reflecting
power. If an automatic arc be used, the regulating mechanism
should be very sensitive, so as to maintain a steady arc of
uniform intensity. The large Schuckert lamps supplied by
Zeiss fulfil this condition. The carbons should be as free as
possible from ash to avoid irregular sputtering. Siemens' A
carbons, cored positives and solid negatives, are suitable.
When a perfectly steady automatic arc is not available, it is
better to use a hand-feed arc lamp than one with an irregular
and unsteady regulating mechanism. With a little practice, it
is not diflicult to maintain a steady illumination with a hand-
feed lamp of the kind supplied by several makers for lantern
purposes.

Next to the arc lamp in convenience, and surpassing it for
the worker who has only an ordinary domestic lighting current

Fig. 65.— Projection
eyepeice.

EXAMINATION OP- PREPARED SECTIONS 1^7

available, comes the Nernst lamp with incandescent filament,
which may be obtained for any usual voltage. The lamp is
placed horizontally, the filament also having a horizontal
direction, and the glass globe is removed. The narrowness
of the luminous source makes it somewhat difiicult to fill the
objective uniformly with light, but this may be overcome by a
suitable arrangement of the condensing lenses, the image of
the filament being thrown slightly out of focus. A lamp with
several parallel filaments is more convenient.

The mercury arc has also been used^ as a source of
light. The Nernst lamp will, however, be generally preferred.

A gas lamp with incandescent mantle is a very convenient
source of light for visual observations, and may also be used
for photography, although it involves a longer exposure than
when electricity is available. A gas flame without mantle is
useless.'

Martens has employed an oxygen-coal gas flame, directed
on to the end of a thin cylinder of magnesia (the rod used as
support for an incandescent gas mantle) so that the actual
source of light is a very small round disc. Precautions are
necessary in regulating the combustion so as to maintain
uniformity,^ and the arrangement is not likely to be adopted
in ordinary laboratories.

For visual observations^ when very intense illumination
is not required, a small glow-lamp may be attached to
the illuminator in a fixed position, thus obviating the
necessity of an optical bench or even of a condensing lens,
and rendering the entire apparatus portable. This arrange-
ment has been adopted in two instances. Messrs. Pellin
construct a modified Le Chatelier microscope in which the
lamp is held in a tube at right angles to the fitting containing
the illuminating prism. The stage is of the usual racking
type. In Stead's workshop microscope,^ there is no stage, the

' H. Le Chatelier, ^itide des Alliages, 1901, 421.

" A Martens and E. Heyn, Milth. k. techn. Versuchs-Ansu, 1899,

17, 73-

' J. Iron Steel Inst., 1908, iii. 22. The instrument is made by Messrs,
Swift and Son.

i68

METALLOGRAPHY

vertical tube being supported by a tripod, so that it may be
applied to a large mass of metal, such as a forging, a small
area only being cleaned and polished. The illuminator is a
plane-parallel plate of glass fixed above the objective, and a
small 4-volt lamp, fed by an accumulator, is held in a tube
at right angles to the optical axis.

When a portable microscope is not used, the microscope
stands at a greater distance from the source of light. Wiith a
large illuminating surface, such as that of a Welsbach mantle,
a simple bull's-eye condenser may be used, mounted on an
adjustable stand, the flat side of the lens being turned towards
the microscope. This gives ample light, and the adjustment

Fig. 66. — Optical bench.

necessary to obtain a field of uniform brightness is very
simple.

With an arc or Nernst lamp, giving a very localized source
ofgreat intensity, a more elaborate arrangement is necessary.
In order to effect the centreing of the lenses, diaphragms, etc.,
employed, some form of optical bench is desirable, on which
the fittings may be made to slide. The necessary fittings for
a complete equipment are : two condensing lenses, an iris
diaphragm, a water-trough for absorbing heat rays, and a
colour filter for obtaining approximately monochromatic light.
All of these should be supported on saddles of similar form,
so as to be readily interchangeable in position.

The most suitable arrangement of the fittings is shown
diagrammatically in Fig. 66. The lens A, of about i6 cm.
focus, converts the rays from the arc or Nernst filament into
a parallel beam, which passes through the water-trough B and
the iris diaphragm C to the second lens E, which focusses the

EXAMINATION OF PREPARED SECTIONS 169

image of the source on to the surface of the specimen. The
apparatus having once been set up in position, it is only
necessary to vary the position of E until the field of the
microscope is of uniform brightness. D is a stand for holding
colour filters. Of these the most generally employed is the
Zettnow liquid filter, a i cm. layer of a green solution com-
posed of 160 g. cupric nitrate, and 14 g. chromic acid, dissolved
in 250 c.c. water. The Gifford screen is rather more con-
venient, consisting of a solution of malachite green in glycerol,
together with a slip of bluish green glass. It is better, how-
ever, to use a series of colour-filters of known absorption, each
of which transmits a definite section of the spectrum. By
inserting one or more of these in the path of the rays, the
contrast may be considerably heightened, with corresponding
advantage to the clearness of the image. Such screens are
made in gelatin, cemented to glass, by Messrs. Wratten and
Wainwright, the A, B, and C screens giving a sufficient range
of colours for all ordinary purposes, whilst the K 3 screen may
be used when it is required to render a coloured surface, such
as that of an alloy which has been heat-tinted, in its true ratio
of luminosities.

The alignment of the fittings is preserved by the form of
the bar forming the base of the optical bench, which has a
triangular or trapezoidal section. Each fitting is as a rule
provided with a means of vertical adjustment. Centreing
should be performed with the iris diaphragm narrowed down
to a small opening.

When an optical bench is not available, it is still possible
to obtain a satisfactory illumination with simple means. Thus,
with a Nernst lamp, an ordinary lantern condenser may be
used in place of the lens A, and a bull's-eye condenser in
place of E. The iris diaphragm may be replaced by a series of
circular stops cut out of blackened card. More trouble is
of course needed in centreing the parts, owing to the absence
of adjusting devices, but accurate centreing is quite possible.
The parts, once placed in alignment, should be attached to
a wooden base-board to maintain their relative positions.

The disposition of the parts of the apparatus hitherto

I70 METALLOGRAPHY

described is the same, whether the microscope is to be used
horizontally or vertically. The latter position is the more
convenient for visual observations, or when drawings are to
be made, but for photographic purposes the former is to be
preferred. It is true that many workers have employed the
vertical camera successfully, but its range is limited, as it
cannot be extended greatly without reaching ao inconvenient
height, and the strain on the eyes is much more severe when
focussing a series of specimens with a vertical than with a
horizontal camera. It is also easier to damp the horizontal
form in such a way as to be unaffected by vibration.

Thecamera maybe placed at right angles to the optical bench,
the optical axis of the latter exactly entering the opening of the
illuminator, or the two may be placed in a parallel position, the
beam falling on a plane mirror placed at an angle of 45° with
its path, and so being reflected into the illuminator. Which of
these dispositions is to be adopted will depend on the shape
of the room at the disposal of the operator, and on the con-
venience of access to the parts. The optical bench, micro-
scope, and camera, may be supported on solid benches
attached to the wall, or on metal tables standing in the centre
of the floor. Tables for this purpose are supplied by several
makers, separate supports being generally used for the optical
bench and the camera. Messrs. Zeiss' pattern of camera
mounted on such a stand is shown in Fig. 67, and is pro-
vided with means of adjustment in a vertical and horizontal
direction. The camera is in two sections, of which the one
on the right of the figure may be used alone when only a
moderate extension is required. With a long camera, means
of focussing the microscope from the screen end should be
provided, either in the form of a long rod with Hook's joint,
or of a rod passing through bearings, having at its end a
pulley over which a cord passes by means of which the milled
head of the fine adjustment may be turned.

Two glass screens are required for focussing purposes.
One of these should be of very fine-grained ground glass, the
other of clear glass. In focussing an object, the ground glass
screen is first inserted, the iris diaphragm being opened far

EXAMINATION OF PREPARED SECTIONS lyi

enough for the field to be illuminated brightly. The specimen
is then moved about on the stage until a suitable field is
selected. It is advantageous to have the size of the plate to
be used ruled in pencil on the ground glass, so that it may be

Fig. 67. — Photo-micrographic camera.

seen at once what portion of the field falls within the limits
of the plate. The iris diaphragm is then closed until the
detail appears clearly up to the edges of the marked area.
When the focus appears sharp, the ground glass screen is
replaced by the clear glass, and the final focussing performed
with the aid of a focussing glass applied to the screen. If a

172 METALLOGRAPHY

colour filter is to be used, this should be inserted before the
final focussing. The screen is now withdrawn and replaced
by the dark slide, and the exposure made in the usual way.
It is advisable to reinsert the focussing screen after exposure,
in order to see whether the necessary movements have caused
any displacement.

Care must be taken to exclude all extraneous light. The
connection between the eyepiece of the microscope and the
camera should not be made rigidly, but
by means of a velvet sleeve with elastic
"collars or of an adaptor of blackened
brass, shown in section in Fig. 68. The
optical bench is most conveniently pro-
vided with a frame of light iron rods,
on which black velvet curtains are hung
freely. It is hardly necessary to add
that the body-tube of the microscope
and the inside of the camera must be
thoroughly dead black. The room should
Fig. 68.-Light-tight ^ave dark bhnds, so that all external
connection. - , , j , , • ,

light may be excluded when high powers

are being used, or when the reflecting power of the specimen
under examination is low.

It is not proposed to give details of the photographic
manipulation, which does not differ from that which is usual
in photography for other purposes. In general work, it will
be found best to use the plates manufactured for " process "
work, which render detail with the necessary clearness. These
plates are comparatively slow, but the exposure necessary with
an arc lamp is nevertheless to be reckoned in seconds with
metals of average reflecting power. In the choice of colour-
filters, plates, and developers, the worker should aim at securing
sufficient contrast, the tendency of beginners being to produce
flat negatives. When accurate colour-filters of known absorp-
tion are used, it is advisable to employ plates rendered sensi-
tive to the whole spectrum, such as the Panchromatic plates
of Messrs. Wratten and Wainwright.

Prints from negatives may be made on bromide or

EXAMINATION OF PREPARED SECTIONS 173

platinotype paper or on one of the numerous gaslight papers
now manufactured. A half-matt surface is to be preferred to a
smooth gloss, although the latter is thought by some to show
more detail in prints from weak negatives.

A permanent record of colours, such as those produced on
heat-tinting an iron or copper alloy, may be obtained by means
of one of the three-colour processes now available. The
Sanger-Shepherd process, which involves the preparation of
three coloured gelatin positives, and their exact superposition,
gives the most brilliant results, but needs much time and
patience.. A simpler process, involving the use of one plate
only, is Lumibre's Autochrome, the plates of which are pre-
pared with mixed starch grains, dyed in three different colours.
This process has found application in metallography in several
quarters.^ All three-colour methods at present yield only
positives, and prints required for reproduction have to be
made by some photo-mechanical process. Many persons will
prefer to make ordinary prints in black and white, somewhat
under-developing, and then to colour the films by hand with
the stains used for colouring lantern slides.

Although, in the great majority of instances, the photo-
graphic method is at once the most convenient and the most
trustworthy means of recording structure, it may sometimes
happen that there is an advantage in making a drawing of
some particular specimen. This may be called for by the
appearance of some structure in a deeply etched alloy, the
features of which cannot be simultaneously brought into focus,
or by the colorations produced by heat-tinting, which are lost
unless some process of colour-photography is employed. Some
patience and a little skill are required to make satisfactory
drawings, but the .task is facilitated by the use of a device,
such as the camera lucida or the Abbe reflector, for projecting
the microscopic imdge on to the drawing paper. The drawing
of the outline is then reduced to mere tracing. Even without
such devices, it is not difficult with a little practice to apply
the left eye to the microscope, and to watch the pencil and
' P. Goerens, Metallurgie, 1508, 6, 19; F. Wust, ibid., 73; E. F.
Law, J. Iron Steel Inst., 1908, i. 151.

174 METALLOGRAPHY

paper with the right, the process then becoming one of draw-
ing from a copy. As in all microscopical work, it is important
that the worker should accustom himself to using both eyes in
turn for observations, the eye not applied to the microscope
being kept open. Neglect of this precaution leads to fatigue
and strain.

As an appendix to the microscopical methods, an interesting
attempt to examine alloys by transmitted radiations, instead
of by reflected light, may be mentioned. This consists in
exposing' a thin section of the alloy to Rontgen rays, the
transmitted rays falling on to a photographic plate. When the
alloy contains crystals of a metal or compound which is opaque
to Rontgen radiations, with a ground-mass consisting mainly
of a less dense and more transparent metal, remarkably sharp
photographs may be obtained.^ With the improved technique
of modern metallography, however, such a method becomes
superfluous, as it naturally fails to reproduce fine detail, and
it is mentioned here on account of its intrinsic interest rather
than as a practical method of investigation.

' C. T. Heycock and F. H. Neville, Trans. Chem. Soc, 1898, 73,
714; Proc. Camb. Phil. Soc, 1898, 9, 417 j Phil. Trans., 1900, 194a,
201.

CHAPTER IX

THE CRYSTALLIZATION OF METALS AND ALLOYS

Whilst the crystallization of solid components from molten
alloys, and of salts from their solutions, are processes of
essentially the same nature^ there is one remarkable difference
in the course of events in the two cases. Salts commonly
separate from their solutions in the form of more or less perfect
crystals, bounded by plane faces, and a small crystal continues
to grow in all directions in which it is free, retaining the same
or a closely similar general form, so that an octahedron
remains an octahedron, the development of additional faces
being only a subordinate feature. Metals and alloys, on the
other hand, only exceptionally form crystals of this kind. As
a rule, a small crystalline nucleus grows almost exclusively in
the direction of certain axes, giving rise to elongated and much
branched forms, the "crystallites" or "crystal skeletons.''
None of the faces attain any great development, even under
favourable conditions of cooling, and curved faces are frequent,
the bounding surfaces being sometimes highly curved, to the
complete exclusion of plane faces.

Exceptions to these rules occur on both sides. A few
inter-metallic compounds, as noticed below, crystallize from
molten alloys in well-developed cubic, rhombic, or hexagonal
forms, bounded by plane faces, whilst it is possible to cause
many salts, and still more readily organip compounds, to assume
the skeletal condition, especially by adding viscous or colloidal
substances to the solution in which crystallization is taking
place.^ Viscosity, however, cannot be resporisible for the
peculiar behaviour of metals, which exhibit the tendency to

' See O. Lehmann, Molekularphysih, vol. x. (Leipzig, 1888), where a
vast amount of information respecting the growth of crystals is collected.

I7S

176 METALLOGRAPHY

form branched skeletons even when deposited electrolytically
from solutions of their salts.

The mode of growth of such a skeleton has been studied
microscopically in a number of substances by Lehmanii. If we
suppose crystaUization to begin at a centre in the midst of the
liquid, a min,ute crystal is first formed. Crystals can only grow
in a supersaturated solution, and as soon as the layer of liquid
adjoining the crystal ceases to be supersaturated, growth is
interrupted, and can only recommence when the concentration
of the layer has been increased by diffusion. Lehmann has
shown that the diffusion is greatest at the most sharply pointed
angles of the crystal, and it is there that the most rapid growth
occurs. Hence, if we suppose the minute nucleus to have the
form of an octahedron, its growth takes place in six directions,
which in this case coincide with the prolongations of the
crystallographic axes. (In the case of a cube, the lines of
growth pass outwards through the corners, and do not coincide
with the crystallographic axes.) The skeletons thus formed
are truly crystalline, and exhibit numerous octahedral faces.
Growth soon sets in at some of the solid angles thus produced,
with the result that a system of secondary axes, at right angles
to the primary axes, is established, and this is succeeded by
systems of the tertiary and higher orders, resulting in the filling
up of the interstices of the skeleton. With salts growing under
ordinary conditions, the " filling-up " process keeps pace with
the axial growth, but it fails to do so in metals. Consequently,
a metallic crystallite growing freely develops axially, the axes
advancing more rapidly than the interaxial matter tending to
complete the crystal. Very fine examples of this mode of
growth are sometimes seen in the crystals of iron projecting
into the internal cavities, or " pipes " of slowly cooled steel
ingots.' It is also common in native metals, the elongation in
the direction of certain axes being very remarkable in the case
of filiform gold, and of arborescent silver and copper, all of
which are elongated in the direction of the octahedral axes.

If the growth of the crystallite is confined in some

' D. Tschernoff, Rev. Univ. Mines, 1880, [ii.] 7, i. 129 ; E. F. Lange,
Mem. Manchester Phil. Soc, 191 1, 55.

A. Surface of cast tin. X 15.

B. Etched surface of cast tin. x 4.

PLATE I.

[.To face page 177.

THE CRYSTALLIZATION OF METALS 177

directions, only a few of the possible skeletal axes are
developed. In an ingot of metal, the outside of which cools
most rapidly, the first crystals start from the surface and grow
inwards, developing unilaterally. The arrangement of axes is
well seen when, as at the free surface of a cast mass, the
crystallites are compelled to grow in a plane, the axes per-
pendicular to that plane being suppressed. The frost-figures
produced by the freezing of deposited moisture on window
frames are a familiar example of such branched or dendritic
growths. Owing to the fact that most metals contract in
freezing, the crystallites which form on the surface of a metal
or alloy are left in relief by the contraction of the mother-
liquor, and this fact makes it possible to photograph the
crystallites without developing the structure by etching.

Plate I., A, represents the surface of d thin mass of tin poured
on to a smooth block of metal, and a similar appearance is cha-
racteristic of most readily fusible metals cast in the same way.

The process of crystallization from any given centre must
ultimately reach a. limit owing to the interference of adjoining
crystallites. The growth of the axes of the crystallite is thus
arrested, and the further solidification of metal takes place
interaxially, axes of higher order being formed until the spaces
are filled with solid matter. If we suppose the original dis-
tribution of the centres of crystallization to have been uniform,
and growth to have taken place with equal velocity in the
direction of each octahedral axis, the solidified mass will be
divided up into exactly similar polyhedra. In any actual
crystallization, the centres are not uniformly distributed, and
the orientation of the axes of neighbouring crystallites is not
the same, so that the polyhedra vary in shape and size.
They may retain a fairly regular polyhedral form, or may be
quite irregular, as is seen in the section of cast tin, Plate I., B,
or neighbouring crystallites may grow to an unequal extent,
giving rise to interlocked and complicated boundaries, often
resembhng the sutures of the cranial bones.'

' An entirely different explanation of the formation of crystal grains,
based on the assumption of foam-structures in the liquid, has been pro-
posed by G. Quincke, see Internat. Zeitsch. Metallographi.:, 1912, 3, 23.

T.P.C. N

178

METALLOGRAPHY

The typical structure of a solidified metal is therefore that
of approximately polygonal " grains," the boundaries of which
are not crystal faces, but are due to interference during growth.
Each grain is, to use the mineralogical term, an "allotriomorphic"
crystal, the outline of which is determined by the presence of
neighbouring grains. The form of the grains is somewhat
different near to a cooling surface, as the crystallites which
start from that surface have time to acquire a considerable
longitudinal extension before their growth is interrupted by
meeting with other skeletons, crystallization in the interior
not setting in until later. The principal growth of the
external skeletons is in a direction perpendicular to the cooling
surface, and the allotriomorphic crystals which result have

consequently an elon-
gated form. This fact is
the cause of the weakness
of castings having a rect-
angular outline. The two
systems of crystals start-
ing from two adjacent
sides meet along a line
bisecting the angle, as
shown in Fig. 69, a. This
line is a line of weakness,
from the ease with which the crystals lying on opposite sides
of it can be separated. The difficulty is avoided by rounding
off the angle as at b.

Metals deposited electrolytically also form skeletons per-
pendicular to the surface of the electrode, and lines of weakness
are similarly produced at the junction of two systems of crystals
starting from the surfaces of a V-shaped groove, making it
possible to divide a sheet of electro-deposited metal along such
a line with ease.^

The nature of the allotriomorphic crystals may be studied
in a solid metal by polishing and etching.^ In such an etched

» W. De la Rue, Mem. Chem. Soc, 1845, 2. 300 ; A. K. Huntington,
Trans. Faraday Soc, 1905, 1, 324.

^ For a method of studying the arrangement of the crystallites in three
dimensions, see C, H. Desch, Prpc. Roy. Phil. Spc. Glasgow, 1912, 107.

Fig. 69.— Crystal growth in ingots.

THE CRYSTALLIZATION OF METALS 179

Fig. 70. — Crystallization from
centres.

section as Plate I., B, the outlines of the grains are rendered
visible by the unequal brightness due to the varying orientation
of the original crystallites. If we represent the crystallites
which first separate, and the
grains formed by them, diagram-
matically as in Fig. 70, and re-
mark that etching with a chemical
reagent develops the internal
structure of a crystal grain by
the production of minute facets,
we see that these facets will vary
in orientation from one grain
to another. Light falling on
such an etched surface will only
be reflected into the tube of the
microscope by such facets as have
the appropriate orientation, and facets having a different orien-
tation will reflect the light outside the instrument, consequently
some grains will appear light and others dark. Such an
effect is easily distinguished from a true difference of chemical
character between the grains by rotating the specimen on the
stage of the microscope. If the difference is one of orienta-
tion only, those grains which appear dark in one position
appear light under a different incidence of the illuminating
beam, and conversely.

Examination under a higher power proves that the
irregularities of surface are due to the exposure of crystalline
facets. Crystals of a pure metal, lightly etched, show the
formation of minute, regularly shaped " etching-pits," which
take the forni of negative crystals. The orientation and form
of these etching-pits may be employed to determine the
crystalUne system to which the substance etched belongs.^
A few typical etch-figures are represented in outline in
Figs. 71-73.

' See especially, H. .Baumhauer, Resultate der Aetzmethode in der
krystallographischen Forschung, Leipzig, 1894 ; and for a short explana-
tion of the theory of etch-figures, V. Goldschmidt, Bull. Univ. Wisconsin,
1904, 3, 23.

i8o

METALLOGRAPHY

Pure iron (ferrite) gives cubical etching-figures (Fig. 71)
which may appear as triangular wedges when the section cuts
the crystal grains at certain angles.^ The pits obtained on a
surface of pure lead ^ are negative cubo-octahedra, and their
outlines are therefore either square or hexagonal, according to
the orientation of the grain (Fig. 72). The microscopical
appearance of such pits is deceptive, they appear on examina-
tion to have the form of small crystals projecting above the

Fig. 71. — Etching-pits in iron. FlG. 73. — Casting-pits in cadmium.

Fig. 72.^Etching pits in lead.

general surface, and it is only by gradually altering the focus
of the microscope that their true nature, as depressions below
the surface, is recognized.

Similar pits, very well defined in outline, are obtained by
casting readily fusible metals, especially cadmium, in contact
with glass or mica. The small air-bubbles enclosed between
the metal and the smooth surface take the form of negative

' J. A. Ewing and W. Rosenhain, Phil. Trans., 1899, 193a, 353 ; E.
Heyn, Zeitsch. Ver. deut. Ing., 1900.

' J. C. W. Humfrey, Phil. Trans., 1902, 200a, 225 ; H. A. Miers,
Min. Mag., 1898, 12, 113.

THE CRYSTALLIZATION OF METALS i8i

crystals, and are the true equivalent of etching-pits.' Such
air-bubbles in cadmium are represented in outline in Fig. 73.
Like the etching-pits, the orientation of these negative crystals-
is constant within any one grain, but varies from grain to grain.

With continued etching, the pits cover the whole surface,
until a stepped arrangement of minute cubes, rhombohedra,
etc., is developed over the whole area. Very perfect cubical
structure has been exposed in this way by etching iron
containing a little silicon.^

The crystalline form of pure metals may be determined by
the observation of crystallites on cooled surfaces " or by means
of etch-figures, when isolated crystals are not available for
measurement. The majority of pure metals crystallize in the
regular system, namely Cu, Ag, Au, Pb, V, Fe, Ni, Pt, Ir, Os,
Pd, Ti, Th, Ge, Hg, Ga, and Cr, as also the non-metals C and
Si. Most of the remaining metals crystallize in the hexagonal
system, the axial ratios being : *

a : c

Beryllium i : 1-5802

Magnesium i : 1-6242

Zinc I : i'3S64

Cadmium i : i'335o

Arsenic i : 1-4025

Antimony i : 1-3236

Bismuth i : i'3035

Tellurium i : i*32g8

Tin is tetragonal, having a: c=i : 0-3857. Several of the
metals, however, exist in more than one crystalline modification,
in which case the forms stable at high temperatures are of a
lower degree of symmetry than those stable at low tempera-
tures, although iron appears to be cubic in the a, /3, and -y

' Ewing and Rosenhain, loc cit.
= J. E. Stead, J. Iron Steel Inst., 1898, i. 145.

' A large number of good photographs of cast surfaces, compared with
etched sections, are given by W. Campbell, Metallurgie, 1907, 4, 801, 825.
* W. Barlow and W. J. Pope, Trans. Chem, Soc, 1907, 91, 1150.

i82 METALLOGRAPHY

modifications.' This, however, is exceptional, and in general
metals follow the same rule as other crystalline substances,
that a polymorphic change with rising temperature results in a
diminution of symmetry.

No essential difference in the mode of crystallization of a
pure metal is introduced by the presence of a second metal in
the liquid alloy, that is, a metal crystallizes from a solvent in
the same way as from its own melt. Thus lead separates from
a lead-silver alloy containing less than the eutectic proportion
of silver in precisely the same way as from molten lead. The
process is also the same when the crystals separating are those
of a solid solution, except that in this case there is a possibility
of a difference of composition between the skeleton first pro-
duced and the layers deposited subsequently. Under ideal
conditions of cooling, these differences of composition are
obviated by the continual readjustment of crystals and mother-
liquor, as described on p. 46, and the solid product is then
indistinguishable microscopically from a pure metal. Under
ordinary conditions of cooling, however, diffusion is unable to
keep pace with crystallization, and the crystals deposited con-
sist of layers of progressively varying concentration. Etching
with a reagent which attacks one of the component metals
more than the other develops a structure in the crystal grains.
For instance, an alloy of equal weights of nickel and copper,
cooled in a small crucible and etched with hydrochloric acid and
ferric chloride, presents the appearance shown in Plate II., A.
The etching reagent acts on copper more readily than on
nickel. The crystallites separating from the molten alloy are
richer in nickel, the metal of higher melting-point, than the
outer layers ; they are therefore less attacked by the reagent,
and appear as light " cores." The boundary between the light
and dark areas is not sharp, the proportion of copper varying
continuously from centre to periphery. Such a structure is
characteristic of all rapidly cooled solid solutions. The
development of cores by etching thus gives information as
to the mode of growth of crystals, corroborating and

' F. Osmond and G. Cartaud, J. Iron Steel Inst., 1906, iii. ; Rev. de
MMallurgie, 1906, 3, 653.

A. Copper 50, nickel 50%. x 86.

■^i^y^ •■

i^.'i, '-r..

V-!'

J,

E. Brass (Cu 70, Zn 30%) annealed, x 86.

PLATE II.

^To face page 182.

A. Copper crystallites in an alloy of silver and copper. X

E. Antimony 85, copper 15%. x S6.

PLATE III.

\To faci pag! 183.

THE CRYSTALLIZATION OF METALS 183

supplementing that derived from the examination of cast
surfaces.

. Twinned crystals are sometimes observed in cooled metals
which have not been subjected to strain, and are then known
as " congenital twins " ; but they are more often observed in
metals and alloys which have been strained and subsequently
annealed. Copper, and those alloys of copper which are
isomorphous with copper, the so-called a solutions, show this
strain-twinning very well. Plate 11., B, represents a specimen
of brass, the «. solid solution of zinc and copper, after being
strained and annealed. The arrangement of parallel laminae,
with alternating orientation, closely resembles that which
characterizes the felspars in igneous rocks. The /3 solutions
do not undergo twinning when strained and annealed.

EUTECTIFEROUS MIXTURES

In the process of crystallization of a constituent from a
eutectiferous alloy, the separation at the cooling surfaces
results in the concentration of the more fusible portions in
the interior of the mass. It therefore happens in many
instances that the whole or greater part of the constituent in
excess forms long skeletons growing from the external surface
inwards. Such highly-developed crystallites, often branched,
are characteristic of many alloys, and are referred to as
" arborescent," " dendritic," " pine-tree " or " fern " structures,
from their general habit. Typical examples are seen in
Plate III., A and B.

In the first of these, representing an alloy of silver and
copper, the crystallites consist of copper retaining a small
quantity of silver in solution. They take the form of long
primary axes, with well-developed secondary, but only a faint
indication of tertiary, axes. The apparent breafcing-up of
the principal axes into isolated masses is due to the surface
of the section cutting the crystallites obliquely, so that the
principal axes lie partly above and partly below the plane of
the section. The isolated masses are really sections through
secondary axes perpendicular to that plane. In Plate III.,
B, which represents an alloy of antimony and copper, the

i84 METALLOGRAPHY

crystallites are of pure antimony, and are composed of
primary, secondary, and comparatively small tertiary axes.

The two groups of crystallites just considered differ
conspicuously in one respect. Whilst the antimony skeletons
are terminated by distinct angles, and may be imagined as
built up of the familiar crystals of antimony, the outlines of
the copper skeletons are smoothly rounded, and there is
no indication of any crystal faces. Both conditions are of
common occurrence in alloys. Whilst antimony and bismuth
are typical of metals which form sharply angular crystals and
crystallites, copper, magnesium, and lead present themselves
only in rounded forms. It would appear that the surface
tension at the boundary where solidification is taking place is
sufificient in the latter case, and insufficient in the former, to
hold in check the tendency of the metallic particles to assume
a definite crystalline arrangement. We are almost entirely
without information as to the conditions of tension prevailing
at the surface of crystallizing metals,

A further example of a similar structure is illustrated in
Plate XIV., A, Chapter XVII.

The Eutectic

A eutectic alloy consists, as has been shown, of an intimate
mechanical mixture of two solid phases, produced by solidifica-
tion at a constant temperature. The pattern produced by the
intergrowth of the two phases takes a great variety of forms,
and the beauty of many microscopic structures is owing to
such patterns. The simple intermixture of small crystals of
the two constituents, generally observed in mixtures of organic
substances, and occasionally in igneous rocks, is rare amongst
alloys. Metallic eutectics commonly show a definite orienta-
tion of one constituent, or else the two constituents are inter-
laminated in curved forms, in which it is difficult to recognize
crystalline structure.

A pure eutectic alloy, solidifying without undercooling,
passes from the liquid to the solid state at constant temperature.
The laminated structure observed suggests that the solidi-
fication of the two phases is not strictly simultaneous, but that

A. Eutectic of copper and copper phosphide. X 200.

B. Eutectic of copper and copper phosphide, x 200.

PLATE IV.

YTo face page 185.

THE CRYSTALLIZATION OF METALS 185

particles of the first and second phases crystallize alternately.'
This process begins, like the crystallization of a pure metal, at
a number of centres, and the interference of the systems leads
to the formation of polyhedral masses or " colonies," " resemb-
ling in general form the crystal grains of pure metals and solid
solutions, but composed in this instance of an intergrowth of
two phases. On etching, as the laminse in neighbouring
grains have generally a different orientation, the boundaries
are developed, and under a low magnification the appearance
is not unlike that of homogeneous crystal grains. Under ob-
lique illumination, many eutectic alloys when etched present an
iridescent lustre like that of mother-of-pearl, owing to the scatter-
ing of the incident light by the laminae. This pearly appearance,
first observed by Sorby in the eutectoid of iron and iron carbide,
hence called by him " the pearly constituent," and by Howe
" pearlite," is characteristic of eutectic and eutectoid alloys.
The arrangement of the laminse which produce it takes many
forms, classified by Stead ' as the curviplanal, the honeycombed
or cellular, and the rectiplanal. Without adhering strictly to this
nomenclature, we may arrange the types of eutectic most fre-
quently observed in approximately the order adopted by Stead.

I. The two solid phases form parallel curved bands, in
which no trace of crystalline outline is to be observed. Pearl-
ite is a good example of this type (Plate XL, B). The narrow
bands of cementite, FcsC, and the broader bands of ferrite, Fe,
exhibit parallel arrangement and may have a considerable
curvature. Another variety of this type is seen in the eutectic
of copper and copper phosphide (Plate IV., A and B). The
parallel bands are involved in complicated patterns. In
A the bands become broader and more irregular as the
border of the grain is approached ; in B a grouping of the
structural elements about a central rod, also consisting of
parallel bands, is observed ; but in neither case is there any
clear indication of crystalline form.

' Recent experiments, however, indicate that the two constituents
separate simultaneously, and that the structure depends on the relative
Velocities of crystallization. R. V.ogel, Zeitsck. anorg. Chem., 1912, 76, 42S.

' C. Benedicks, Internat. Zeitsch. Mitallagraphie, 1911, 1, 184.

» Proc. Cleveland Inst. Eng., 1900, Feb. ; Metallographist, 1902, 5,

i86 METALLOGRAPHY

2. The bands have a toothed or indented form, the more
or less regular undulations being repeated in neighbouring
laminae. This arrangement is seen in the eutectic of lead
and tin.

3. The toothed bands of Group 2 are sometimes observed,
under higher magnification, to be composed of very numerous
rods parallel to one another and normal to the direction of the
laminae. This is seen in the eutectic of copper and silver
(Plate III., A). The short rods of copper have the same orienta-
tion over considerable areas, and when cut transversely appear
as dots ; whilst an oblique section, such as that on the right-hand
side of the figure, appears as if crossed by parallel lines. The
eutectic of copper and copper oxide has the same structure,
and a third example is the eutectic of nickel and nickel phos-
phide. In all these alloys we must regard the rods as crystal-
lites having parallel orientation.

4. The third class passes through gradations into the
honeycombed structure, well represented by the cementite-
martensite eutectic of white cast-iron (Plate XIV., B). Whilst
the general parallelism and similar orientation throughout
certain areas is obvious, the crystalline nature of the grouping
is less easily observed than in the preceding group.

5. Boxed forms, characterized by the formation of simple
polyhedral crystallites divided transversely by numerous parallel
septa, are most typically developed in the alloys of bismuth.
Plate v.. A, represents an alloy of bismuth and tin, and an angle
of such a crystallite is seen extending to the centre of the field.
Indications of the same structure in an incomplete form appear
in the lower half of Plate V., B, representing an alloy of
bismuth and lead.

6. Occasionally tlje eutectic is composed of plane plates of
one constituent disposed in a radiating manner, the second
constituent occupying the interstices. The eutectic alloys of
silver and lead and of lead and antimony yield thin flat plates
on deep etching. Such a radial arrangement of plates resembles
very closely the spherulitic structures observed in many igneous
rocks, especially those formed by the slow devitrification of an
originally glassy magma. A comparison of spherulites and

A. Eutectic alloy of bismuth and tin.

X 200.

B. Eutectic alloy of bismuth and lead. x 200.
PLATE V.

[ To face p(igi^ 1 86.

w" ,-■,£■ V" y'v ,

A. Alloy of antimony and Cu^Sb. X 200.

r^^^y.

,"!;

L\^>

ix^"

B. Eutectic alloy of antimony and Cu„Sb. X 200.
PLATE VI.

\_To fai£ page 1S7.

THE CRYSTALLIZATION OF METALS 187

eutectic alloys suggests a great similarity in the process of
crystalline intergrowth in the two cases.^

7. The eutectic alloy of antimony and copper antimonide
occasionally presents a structure which throws much light on
the process of crystallization of eutectics.* In alloys containing
an excess of antimony above the eutectic proportion, the anti-
mony particles, instead of forming continuous bands, may
appear as nlinute crystals of similar orientation (Plate VI., A).
The orientation is obviously determined by that of the original
crystal skeleton of antimony near to which they are grouped.
The entire similarity of the principal and the subsidiary crystal,
lites proves that the antimony alone determines the orientation,
the copper antimonide (dark in the photo-micrograph) merely
acting as a filling material. At a greater distance from the
principal skeletons, the structural arrangement (Plate VI., B) is
less easily deciphered, but the long, tapering laminae may be
recognized as being also crystallites of antimony, cut at a very
oblique angle. A transition to curved forms is noticeable here
and there, and some specimens of this eutectic, free from excess
of either constituent, have in fact the typical curved banded
structure of Group i, without any indication of crystalline
form.'

A comparison of Groups 3 and 7 suggests that the pattern
of the eutectic is determined principally, if not exclusively, by
the crystalline habit of one of the constituents, the other
behaving to some extent as a plastic substance, yielding and
adapting itself to its growth. This conclusion has been arrived
at by several investigators. It remains as yet undetermined to
which physical property this action of one of the constituents
is due. It has been suggested by Stead that the harder con-
stituent has a determining influence on the crystalline habit ;
by Rosenhain, that it is the constituent present in the larger

' J. H. Teall, Quart. J. Geol. Soc, 1901, S7, Ixii. For the charac-
teristics of spheiulitic crystallization in rocks, see W. Cross, Proc. Phil.
Soc. Washington, 1891, 11, 411 ; J. P. Iddings, ibid., 445.

^ Communication by the author to Section Wi of the International
Congress of Applied Chemistry, 1909.

' J. E. Stead, J. Soc. Chem. Ind., 1898, 17, illi j A. Baikoff, BulU
Soc. d' Encouragement, 1903, 103, 626.

iS8 METALLOGRAPHY

proportion by volume in the eutectic alloy ; and by the author,
that the power of orientation, as shown by the ability to form
extended crystal skeletons, is the determining factor.

As the composition of the alloy departs in the one direction
or the other from the eutectic proportion, the structure
naturally changes. A small deviation may fail to bring about
the appearance of the constituent in excess in the form of
distinct crystals or crystallites, but is often rendered visible by
a coarsening of the eutectic structure, first apparent at the
margin of the grains. Such a thickening of the structural
elements of the eutectic, due to a minute excess of copper
phosphide over the eutectic proportion, is seen in Plate IV., A.

If the departure from the eutectic composition be any
greater, crystallites of the constituent in excess make their
appearance, at first as small isolated masses, becoming united
to form skeletons or larger crystals with further change in the
composition. A comparison of Plates III., B ; VI., B ; and
VII., A, will illustrate the variations of structure as the eutectic
point is passed. In Plate III., B, the antimony is in excess,
and its long crystal skeletons are separated by the eutectic of
antimony and copper antimonide. Plate VI., B, represents the
pure eutectic alloy, whilst in Plate VII., A the long dark
crystals are those of the compound CuaSb, now the constituent
in excess.

Inter-metallic Compounds

An alloy consisting entirely of an inter-metallic compound
Crystallizes in the same manner as a pure metal. The typical
structure of such an alloy is consequently that of a uniform
substance, divided into polyhedra presenting themselves as fine
polygonal boundaries in an etched section. Similarly, the
structure of a solid solution composed of an inter-metallic com-
pound and one of its components does not differ from that of
the solid solutions previously described. When, however, the
compound is accompanied by a eutectic, its development often
differs in a marked degree from that of a single metal. The
tendency to form crystals with plane faces, like the crystals of
salts deposited by an aqueous solution, instead of skeletons, is

A. Antimony 75, copper 25 %. x 86.

B. Alloy of tin and antimony, x 20.
PLATE VII.

\_To face page 1S8.

THE CRYSTALLIZATION OF METALS 189

much more pronounced among inter-metallic compounds than
among the metals. The crystallographic constants of a few
only of such compounds have been observed, but a rhombic
habit certainly predominates. In general, it may be said that
the symmetry of the compound is lower than that of its com-
ponents. Cubic forms are rare, and such apparent cubes as
are observed are probably less simple crystals, having a pseudo-
cubic habit. An instance of this is seen in Plate VII., B, the
cubic or pseudo-cubic crystals of which consist of a solid
solution approximating to the composition SnSb. A rhombic
outline is much more commonly observed in etched sections,
among the compounds exhibiting this habit very distinctly being

AgMgs, AleCo, AlgCu (Plate VIII., A), SbaZus, and AuMga

The constants of a few compounds have been determined,
amongst them being

CdgSba rhombic, a : 3 : f = o'759i : i : 0*9687 ;
FeSb2, rhombic, a:b : c = 0*5490 : i : i'i224 ; '

NiSbj hexagonal, a : c = i : i"294o ;
SbZn, rhombic, a: b : c = 07609 : i : o'9598 ;
AlsFe, monoclinic, a:b : c = i'S4i3 : i : i'9i58, |3 io7°4i';2

Cd2Na and MgaSn are described as regular, the former forming
combinations of the octahedron and regular dodecahedron, and
the latter octahedra.*

When a compound of the above type is present in large
quantity, the small crystals may unite to form skeletons. This
is the case, for instance, with FeSbg, which crystallizes in simple
rhombs if present in small excess over the eutectic composition,
but in larger excess forms characteristic skeletons.

Crystallization from Solid Solution

An alloy composed of two solid solutions frequently
resembles, especially under low magnifications, one consisting

' N. S. Kurnakoff and N. Konstantinoff, Zdtsch. anorg. Chem., 1908,
68, I.

' P. Groth, Chemische Krystallographie, Teil I., Leipzig, 1906.
' P. von Suschtschinsky, Zeitsch, Kryst. Min., 1904, 38, 264,

igo METALLOGRAPHY

of crystals of one constituent, surrounded by a eutectic.
Higher magnification, however, fails to develop a eutectic
structure in the ground-mass, which remains homogeneous on
etching, or at most exhibits the zonal variation of composition
characteristic of solid solutions in a state of imperfect equi-
librium. The tin-antimony solid solution in Plate VII., B, is
a ground-mass of this kind; the etching has developed the
crystal boundaries, which appear as irregular lines, but there
is no indication of any eutectic structure.

If the alloy solidifies in the first instance to form a single
solid solution, from which a new constituent crystallizes on
further cooling, the first appearance of the new crystals takes
place, in the majority of cases, at the bounding surfaces of the
original polyhedra. An alloy of 62 per cent. Cu and 38 per
cent. Zn, for example, solidifies as a single solid solution, /3
(p. 62), and at about 700° crystals of u, separate, the solubility
curve of a in /3 being crossed at that point. The first separa-
tion of a takes place at the boundaries, and the crystals there
formed grow inwards towards the centre of the polygonal
grains. Plate VIII.., B, representing an aluminium bronze, is
a typical example of the structure of such an alloy, an o/3
aluminium-copper alloy being, in fact, indistinguishable from
an a^ zinc-copper alloy in which the structural elements are
present in the same proportions. Plate IX., A, represents an
aj3 copper-zinc alloy, in which the boundaries are curved.
The section is cut transversely through a rolled bar. Further
examples of structures produced in the breaking-up of solid
solutions occur amongst the alloys of iron and carbon, and will
be noticed in Chapter XVII.

Crystallization in Ternary Systems

The process of crystallization of a ternary alloy is precisely
the same, in its initial stages, as in a binary system. Both
binary and ternary eutectics may occur as micrographic con-
stituents, the former generally presenting itself as a border sur-
rounding the primary crystals, whilst a ternary eutectic occupies
the intercrystalline spaces. A simple case is illustrated in

A. Copper 55, aluminium 45 %. x S6.

\_To face page 190.

Copper 60, zinc 40 %. Transverse section of rolled bar. X 86.

A??

^^~

B. Alloy of bismuth, lead, and tin. X 200.
PLATE IX.

\_To fiue page 191.

THE CRYSTALLIZATION OF METALS 191

Plate IX., B, which represents an alloy of bismuth, tin, and
lead, the bismuth being in excess. The primary crystals of
bismuth, of which two appear in the upper part of the photo-
graph, are surrounded by the characteristically banded eutectic
of bismuth and tin. The orientation of this honeycombed
structure is determined by that of the primary crystals. The
ternary eutectic, the structure of which is not very clearly
resolved, appears as a dark, almost black, mass, in the lower
part of the photograph. If more completely resolved, it would
be found to consist of the three metals in a finely inter-
laminated condition.

An interesting case, which presents itself in technical
practice, is the disturbance of the equilibrium of two solid
solutions produced by the introduction of a third metal. An
important class of alloys of copper and zinc consists of conglo-
merates of two solid solutions, a and ;8. The addition of
small quantities of tin, silicon, manganese and other elements
to such alloys is a common practice, and the study of the
alterations of structure involved has given interesting results.*
The third metal dissolves, up to a certain limit of concentra-
tion, in the two solid solutions, any excess above this limit
separating, in combination with some of the copper, as a
third constituent. The eutectic relations in the system are
as yet unknown. As long as the proportion of the third
metal is below the limit of saturation, its effect is to displace
the limits of the two solid solutions, in other words, the added
metal replaces a certain quantity of zinc, usually much exceed-
ing its own weight. For example, tin replaces twice its
weight of zinc, and an unsaturated ternary alloy of copper,
zinc, and tin, is indistinguishable microscopically from a binary
alloy containing a higher percentage of zinc. The physical
and mechanical properties of the ternary alloy closely approach
those of the binary alloy which it resembles in structure. We
may therefore speak of a " fictitious " value for the zinc con-
tent, bearing a definite relation to the quantity of added metal.
The value may be calculated in the following manner :—

Each of the elements considered has a definite coefficient

' L. Guillet, Rev. de Mtiallurgie, 1906, 3, 243.

192 METALLOGRAPHY

of equivalence t, i per cent, of it replacing t per cent, of zinc,
the quantities being recalculated to loo per cent. These
coefificients are determined by experiment, the values determined
by Guillet being ;

Al
Sn
Mg
Si

6 Pb . . . I

2 Fe . . , o*9

2 Mn ... 0-5
10

Antimony, cadmium, and phosphorus are dissolved to so small
an extent as to be incapable of displacing an appreciable
quantity of zinc.

An alloy containing 68 "47 per cent. Cu, 27 '03 per cent. Zn,
and 4"5o per cent. Al, has the same structure and nearly the
same properties as one containing 56 per cent. Cu and 44
per cent. Zn :

"fictitious" Zn = (^7-03 + 4-5° X 6) 100 ^
68-47 + 27'03 + 4"5o X 6

In general, if B is the actual and B' the " fictitious " percentage
of zinc, and q the quantity of the added metal,

-g, _ (B + tq) 100
100 + tq

If we know the values of B and q, and determine B' by a
microscopical examination, and if necessary by a planimetric
analysis of the alloy, t, the coefficient of equivalence, is given
by the formula—

^^ 100 (B' - E)
q (100 - B')

And in a similar manner, B' or q may be calculated if the
other quantities are known.

Taking aluminium, with its high coefficient of 6, as an
example, its action in modifying the structure of the copper-
zinc alloys is as follows. An alloy is converted by the addi-
tion of aluminium into one having the properties and structure
of an alloy of higher zinc content, in accordance with the
formulae given above. An alloy consisting of the a constituent

THE CRYSTALLIZATION OF METALS 193

only may thus be converted into an a + j8 alloy, an a + /3
into a /8, or j8 into a ;8 + y alloy. A separate aluminium
constituent does not appear until 10 per cent. Al has been
added.

Manganese is' dissolved up to 8 per cent. ; the solubility of
the other elements enumerated is less. When more than one
element is introduced into a copper-zinc alloy, the constitution
of the product may be calculated by the above formulae, only
replacing tq by %tq, provided that the quantities are insufficient
to cause the separation of new constituents,

NON-METALUC ELEMENTS IN ALLOYS

The relations of the non-metals to each other and to the
metals fall for the most part outside the scope of this work,
but a few non-metals are capable of entering into associations
which may be classed as true alloys. For example, steel and
cast-iron are to be considered as alloys, although carbon is a
non-metal. Its solutions in iron, however, have an entirely
metallic character, as has the carbide, FcjC, which crystallizes
in hard, glistening plates. Manganese carbide is also metallic,
but the easily decomposable carbides of aluminium, calcium,
etc., have no metallic properties, and do not form alloys.
Silicon is another alloy-forming element, and in fact may be
considered for metallographic purposes as a metal, from the
general resemblance of its behaviour in association with metals
to that of a highly infusible metal. Even phosphorus unites
with iron, copper, and tin, to form metallic phosphides. The
structures represented in Plate IV. are characteristic of metals,
and the compound CU3P, which is one of the constituents of
the alloy, has all the properties of a hard, brittle inter-metallic
compound.

Mixtures of this class do riot retain the metallic character
throughout the whole range of composition. When the pro-
portion of the non-metallic component exceeds a certain
limit, the mixtures have no longer the properties of alloys.
It is often impossible to prepare the complete series, owing to
the insolubility of the non-metal when a certain limit is
T.P.C. o

194 METALLOGRAPHY

exceeded (thus carbon cannot be dissolved in iron at any
accessible temperature to a higher concentration than 7 per
cent.) or to the high dissociation pressure of the combinations
formed, causing the escape of the volatile component when
the mixture is heated under atmospheric pressure. Thus the
systems formed by iron, nickel, manganese, and copper with
phosphorus, have only been examined over a range of com-
position in which the phosphorus is less than about 15 per
cent., this being the maximum quantity retained at the melting-
point under atmospheric pressure.

Oxygen less often needs consideration as an alloy-forming
element. Copper eagerly absorbs oxygen when fused, the
cuprous oxide produced dissolving in the metal to form a
homogeneous solution. The freezing-point diagram is re-
produced later (Fig. 76). Nickel behaves in an exactly
similar manner.^ Other oxidizable metals absorb oxygen, but
the oxide formed is insoluble, and merely forms a dross on the
surface. The behaviour of silver is unusual. Molten silver
dissolves considerable quantities of oxygen, but rejects it at
the moment of freezing, the escape of the dissolved gas giving
rise to the well-known phenomenon of " spitting " observed
when silver ingots are cooled. The behaviour of the gases
dissolved in metals is of great interest. The effect of hydrogen
on iron has been studied to a limited extent,^ whilst that of
nitrogen on the same metal has been the object of numerous
studies, on account of a supposed embrittling eifect of nitrogen,
even in small quantities, on steel.^ Very interesting results
have been obtained by Sieverts,* who finds that the solubility
of gases in metals generally increases with the temperature,
with a sudden increase on passing from the solid to the
liquid state. The solubility is proportional to the square root

' R. Ruer and K. Kaneko, Metallurgie, 191 2, 9, 419.

' W. C. Roberts-Austen, 5th Report to Alloys Research Committee,
1899, 42 ; E. Heyn, Stahl u. Eisen, 1900, 20, 837.

' H. Braune, Rev. de Mitallurgie, 1905, 2, 497 ; H. Le Chatelier^
ibid., 503, 898; H. Tholander, Jernkontorets Ann., 1(888, 425; C. E.
Stromeyer, y. Iron Steel Inst., 1909, i. 404.

* A. Sieverts and W. Krumbhaar, Ber., 1910, 43, 893 ; Zeitsch.
physikal. Ckem., 1910, 74, 277; A. Sieverts, ibid., 191 1, 77, 591.

THE CRYSTALLIZATION OF METALS 195

of the pressure. Hydrogen is dissolved in this way by copper,
nickel and iron, but is insoluble in cadmium, thallium, zinc,
lead, bismuth, tin, antimony, silver and gold. Its solubility
in copper is unchanged by alloying with silver, increased
by alloying with nickel or platinum, and diminished by
alloying with gold, aluminium or tin, the effect being pro-
portional to the quantity of the added metal as long as no
change in constitution takes place. Sulphur dioxide is in-
soluble in solid, but very soluble in molten copper. On
freezing, a large part of the dissolved gas is retained
mechanically.

Mechanical Enclosures

The microscopical examination of alloys frequently reveals
enclosures of substances which have been mechanically
entangled by the fused alloy, and have not formed a part of
the homogeneous system. Some practice is required to dis-
tinguish these accidental constituents from phases sharing in
the equilibrium. Chief among them are the masses of slag
enclosed in cast ingots. Steel commonly contains minute
masses of silicate slag, in which a crystalline structure may be
developed, and of manganese sulphide, the latter forming
globules of a characteristic dove-grey colour. The form of the
globules is dependent on the treatment to which the me'tal has
been subjected, rolled masses exhibiting an elongation of all
enclosures in the direction of rolling. The examination of
micro-sections for slag masses is of considerable importance in
technical practice, on account of their influence on mechanical
properties. Ferrous sulphide, for example, forms films separ-
ating the crystal grains, and is consequently a source of
weakness when the steel is worked. The addition of manganese
converts the sulphur into manganese sulphide, which collects
in round or oval globules, which, as they do not interrupt the
continuity of the metalUc crystals, have less influence on the
strength.*

' The constitution of the sulphide enclosures in steel is described by
G. Rohl, Iron Steel Inst. Carnegie Schol. Mem., 1912, 4, 28.

196 METALLOGRAPHY

The slag in wrought iron occurs in narrow, thread-like
masses lying in the direction of rolling or working, so that in
a longitudinal section they appear as narrow lines, or in a
transverse section as round dots.

Crystalline enclosures are of less frequent occurrence. Well-
defined crystals of stannic oxide are met with in bronzes which
have absorbed oxygen during melting,^ as they do not separate
readily from the molten metal. Minute crystals of corundum
may also be observed in metals and alloys prepared by
the aluminothermic method. Crystal skeletons of manganese
sulphide have been observed in large steel ingots, and have the
appearance of having solidified before the mass of the metal.^
This points to a considerable solubility of the sulphide in steel
above the melting-point.

' E. Heyn and O. Bauer, Zeitsch. anorg. Chem., 1905, 45, 52.
^ A. A. Baikoff, Ann. Inst. Polyt. St. Petersburg, 1907, 8, 289. See
S. Wologdine, ^iW. de Mitallurgie, 1908, 5, 177.

CHAPTER X

UNDERCOOLING AND THE METASTABLE STATE

Hitherto only systems in a state of stable equilibrium have
been considered, the assumption being made that each new
phase makes its appearance at the temperature at which it is
theoretically formed. This condition is by no means always
fulfilled in practice. The molecular inertia often causes a
body to remain in a condition differing from that of maximum
stability, until some external circumstance causes rearrangement
to take place. The simplest instance of this is seen in the
formation of the solid phase in a cooling homogeneous liquid.
When pure water is cooled below its freezing-point in a clean
vessel, the separation of ice does not take place when the
temperature reaches o°. On further cooling, a point is reached
at which crystallization suddenly sets in, and the temperature
rises rapidly to o°, freezing then proceeding steadily. This fact
was observed by Fahrenheit in the eighteenth century. Fused
metals exhibit the same behaviour, and the phenomenon is
now known to be a general one, the terms undercooling or
superfusion being applied to it.-'

" The effect is shown in the cooling curve, which assumes
the form shown in Fig. 74, the normal curve, in the absence of
undercooling, being indicated by the dotted straight line. When
the undercooling is considerable, and the mass is losing heat
rapidly to its environment, the latent heat liberated may be

' The term " undercooling " (Germ. Unterkiihlun^ will be employed
in what follows, since, as will be shown, the phenomenon occurs at other
changes of state besides that of freezing, to which alone the term "super-
fusion " (Fr. sur/usioti) is applicable.

197

198

METALLOGRAPHY

Fig. 74.— Effect of
undercooling on
cooling curve.

insufficient to raise the temperature to the true freezing-point
A, and the summit of the curve will therefore indicate too low
a freezing-point. For this reason, under-
cooling is to be avoided in the taking of
cooling curves.

In the operation of cupelling gold, the
globule of molten gold under certain con-
ditions emits a brilliant light at the moment
of solidification. The development of
heat at this point was at first ' attributed
to a chemical reaction between dissolved
oxygen and metals present as impurities.
A careful study of this "flashing" or
"dclair," proved, however, that the phe-
nomenon is due to undercooling.^ The
gold must be in a clean state to show the
efiect, and must not be shaken during
cooling. Its temperature falls below the freezing-point until
crystallization sets in, and the development of heat causes the
glow. If the globule is touched with a gold wire when it has
cooled to the freezing-point, freezing takes place normally
without any flashing.

Soon after the first observation of undercooling by
Fahrenheit, it was observed ' that saturated salt solutions
behave in a similar manner, it being possible to cool them
below the point at which crystallization should take place, the'
separation of crystals setting in suddenly when further cooled,
the temperature at the same time rising to the crystallizing
point. What has been said of pure substances therefore also
applies to the separation of one component from a mixture.
Solutions cooled in this way below their proper crystallizing

' A. Levol, Ann. Chim. Phys., 184S, [iii.] 15, 55.

' A. D. van Riemsdyk, ibid., 1880, [v.] 20, 66 ; Chem. News, 1880,
41, 126, 266.

' J. T. Lowitz, CrelVs chem. Ann., 179S, 1, 3. A full history of the
subject of undercooling and supersaturation is given by W. Ostwald,
Lehrb. II. 2, i. 704 ff. Accurate measurements of the extent of under-
cooling in various metals and alloys were made by W. C. Roberts-Austen,
Proc. Roy. Soc., 1S98, 63, 447.

UNDERCOOLING AND METASTABLE STATE 199

points are said to be supersaturated, and, the sudden crystalliza-
tion of a supersaturated solution of sodium sulphate in water
is a common lecture experiment.

If the viscosity of the liquid is high, the undercooling may
be carried very far. Many organic substances — ^for instance,
papaverine — become very viscous in the undercooled state, and
on reaching the ordinary temperature consist of a glassy mass,
which may be preserved indefinitely without crystallization
taking place. Ordinary glasses are mixtures, chiefly of silicates,
the viscosity of which in the undercooled state is so great that
crystallization entirely fails to occur during cooling in the
ordinary process of manufacture. If subsequently treated in
such a way as to facilitate crystallization, as by prolonged heating
at a temperature sufficient to lessen the viscosity to a marked
degree, " devitrification " takes place, crystals corresponding
with the stable solid phases making their appearance. Glassy
volcanic rocks, such as obsidian, are also formed from an under-
cooled magma, and all stages of their devitrification are found
in nature.

The formation of glasses, that is, of rigid non-crystalline
masses, by undercooling, has not been observed in metals or
alloys, the viscosity of which is in fact low,^ In the cooling of
an alloy, the formation of a solid phase always takes place,
although the temperature may be considerably below that
corresponding with the true freezing-point.

Investigations of undercooled liquids have shown that the
region below the solubility curve should be divided into two
parts. In the first, immediately below the curve representing
the equilibrium of the solid and liquid phases, crystallization
does not take place spontaneously, but only when a crystal of
the solid substance (or of a solid isomorphous with it) is
introduced. At a lower temperature, crystallization begins

' Very few determinations of the viscosity of molten metals exist.
Koch, Ann. Physik, 1881, 14, i, made a. series of measurements with
mercury, and C. E. Fawsitt, Trans. Chem. Soc, 1908, 93, 1299, obtained
figures for mercury, tin, lead, and bismuth, varying from one to three
times the viscosity of water. It is a familiar fact that metals, even of high
melting-point, form mobile liquids, which may be poured like virater if free
from dross.

200 METALLOGRAPHY

spontaneously, shaking or other mechanical disturbance being
sufficient to initiate it. On the suggestion of Ostwald, these
regions have been termed metastabk and laUle respectively.
It was long a matter of doubt whether there was any true
boundary between the metastable and labile regions, it having
been argued that the apparent existence of a region in which
crystallization would not occur spontaneously was only due to
the small number of crystal centres, corresponding with a small
undercooling.^ This has been disproved by the exhaustive
investigations of Miers and his collaborators." It has now
been shown that spontaneous crystallization, that is, the
appearance of the solid phase without the previous intro-
duction of a particle of the solid (" inoculation ") does not take
place above a certain definite temperatul-e, which is character-
istic of each substance. The same holds good of mixtures.
Below the solubility curve, and approximately parallel with
it, lies a curve representing the temperature at which each
solution begins to deposit crystals spontaneously, without
inoculation with a crystal. To this curve the name of super-
solubility curve has been given.

Most supersolubiUty measurements have been made with
aqueous solutions of salts. Metallic alloys have not yet been
investigated from this point of view, but the mixtures of two
organic substances, betol and salol, have been fully investi-
gated,^ and it is practically certain that the diagram constructed
from the data thus obtained is typical of the conditions which
must prevail in alloys. The diagram is therefore reproduced
(Fig. 75). It will be seen that the two compounds form a
simple eutectiferous series, no solid solutions being formed.
The freezing-point curve therefore consists of two branches,
AC and BC, intersecting at the eutectic point C. The two
supersolubiUty curves, DF and EF, also intersect at a point, F,
to which the name hypertedic point has been given. It will be

• G. Tammann, Zeitsch. physikal. Chem., 1898, 25, 442.

' H. A. Miers and F. Isaac, Trans. Chem. Soc, 1906, 89, 413 ; H.
Hartley and N. G. Thomas, ibid., 1906, 89, 1013 ; H. Hartley, B. M.Jones,
and G. A. Hutchinson, ibid., igo8, 93, 825.

' H. A. Miers and F. Isaac, Proc. Jioy. Soc, 1907, 79a, 322 ; F. Isaac,
Hid., 1910, 81a, 344.

UNDERCOOLING AND METASTABLE STATE 201

noticed that the abscissae of these two points are not neces-
sarily the same, so that the eutectic and hypertectic mixtures
need not have the same composition.

The area ACBEFD is the metastable region. Mixtures
represented by points falling within its limits are liquid, and

100

^

70

K

v 0,

eo

\i ^

\

so

i\

\^

^r

4-0

30

V

'X

'^

20 <

^^

<^

\w

10

L/

7^

~^v

10 20 JO 4-0 SO 60

Fig. 75. — Betol and salol.

30

90 100 %
SaZoL

only deposit crystals if the appropriate solid phase is intro-
duced. Below the line DFE is the labile region, in which
crystallization can occur spontaneously, although it may some-
times fail to occur when either of the supersolubility curves is
crossed. The significance of the four curves may now be
illustrated by the consideration of a few possibilities.

A mixture represented by the point is cooled, the access
of solid matter being prevented. When the point/ is reached,
the solid phase being 'absent, crystallization does not take place,

202 METALLOGRAPHY

and the mixture remains liquid until the temperature has fallen
to q. The supersolubility curve of betol, or more generally, of
the substance M, has now been cut, and crystallization takes
place spontaneously if the mixture is shaken. The composi-
tion of the part remaining liquid is now represented by a
point travelling down DF, until F, the hypertectic point, is
reached. This being the point of intersection with the second
supersolubility curve, the second solid, N, begins to separate,
and the whole of the mother-liquor solidifies at the constant
temperature of the point F.

Taking the same mixture o, we may now assume that on
reaching the temperature /, a crystal of M is introduced.
This crystal grows, and the point representing the composition
of the liquid moves along /C. When the eutectic point C is
reached, however, the eutectic mixture does not solidify as a
whole, as it is metastable as regards the substance N, the
separation of M therefore continues in the direction CH. The
temperature then falls until the curve EF is cut, crystallization
of N begins, and continues until the hypertectic point is
reached. The curve BC may similarly be prolonged into the
metastable region, as shown by CG.

In the absence of mechanical disturbance, mixtures may
even be cooled below the temperatures indicated by the curves
DF and EF without crystallizing. Thus the mixture o may
be cooled, under favourable conditions, below q, but crystalliza-
tion is sure to begin before the undercooling has proceeded
very far into the labile region, unless prevented by increasing
viscosity. The heat developed causes the temperature to rise
to q, and freezing then proceeds. Under similar conditions,
however, the phase N may also fail to appear when F is
reached, so that the labile prolongation FK may be experi-
mentally realized. The curve EF may be similarly prolonged
as at FL.

The formation of crystals on the upper curves takes place
with comparative slowness, and the temperature may therefore
fall low enough for the supersolubility curve to be intersected,
before the separation of solid is complete. In that case,
a further crystallization of the same solid phase, but now

UNDERCOOLING AND METASTABLE STATE 203

proceeding rapidly from a large number of centres, may take
place. Miers and Isaac found it easy to distinguish by
inspection alone the crystals formed in the metastable region
from the shower deposited on reaching the supersolubility
curve.

It is clear from this description that the eutectic point C
may entirely fail to appear on the cooling curve of a mixture.
In one case, that of a mixture containing go per cent, salol and
10 per cent, betol, Miers and Isaac were even able to observe
four freezing-points, none of which was the eutectic point. Re-
presenting such a mixture by r, the upper salol curve is crossed
at s, at which point salol crystals may be obtained by inocula-
tion. Crystals of betol are obtained at u by inoculation with that
compound^ whilst labile showers of the two components may
be obtained spontaneously by cooling to / and v respectively.

When one or more of the components is capable of exist-
ing in polymorphic forms, the conditions become yet more
complicated, since a metastable modification may crystallize
from the Hquid. This has been observed in mixtures of
naphthalene and chloroacetic acid, the latter of which may
crystallize in three modifications, each of which has its own
supersolubility curve. ^

It is not known how far below the solubility curves the
metastable hmit is to be placed in the case of alloys, but
microscopical evidence favours the view that the conditions pre-
vailing closely resemble those just described. The cooling of
a liquid metal or alloy below its freezing-point is a familiar
fact, and even the addition of a crystal of the solid phase
sometimes fails to remove the evidence of undercooling from
the cooling curve. This is no doubt due to the comparatively
slow growth of crystals in the metastable region, allowing the
temperature to fall to the metastable limit before equilibrium is
reached, a rapid separation of crystals from the labile liquid
then taking place, with a corresponding rise in temperature.

The prolongation of a freezing-point curve into the meta-
stable region, as at CG (Fig. 75), is very frequently observed.
The effect on the cooling curve is to cause the eutectic arrest
' H. A. Miers and F. Isaac, Phil. Trans., 1909, 209a, 337.

204 METALLOGRAPHY

to come too late. When the method of thermal analysis is
adopted, it is necessary to prevent this undercooling of the
eutectic by inoculation with the second component, N, when
the temperature corresponding with C is reached. In the
absence of such inoculation, the duration of the eutectic arrest
is diminished, causing inaccuracy in the eutectic time curve.
The effect on the microscopic structure is to cause the crystals
of M to occupy too large an area, the area of eutectic being
correspondingly lessened. Undercooling may even proceed
so far that the eutectic structure is absent from the section of
the solidified alloy, especially when the constituent which
crystallizes first is in large excess. As an example, the
solutions of cuprous oxide in copper, which behave as true
alloys, may be taken. The freezing-point curve, up to 9 per
cent. CuaO, is shown in Fig. 76.^ When the copper is in excess
of the eutectic proportion, the freezing usually takes place nor-
mally, and the sections obtained show areas of copper and of the
Cu — CugO eutectic.^ The case is different, however, when the
mixture is richer in oxygen, so that crystallization commences
on the second branch of the curve. If slowly cooled, the
crystals of CuaO continue to grow after the eutectic point is
reached, and this process continues until nearly all the oxide
has been removed from the liquid alloy. At this point, the
supersolubility curve of copper is probably intersected, and
practically pure copper soUdifies around the skeletons of
oxide.

An effect which is seen in the microscopic sections of
many alloys may possibly be due to the same cause. This is
the appearance of a band of the constituent N surrounding the
crystals of M which had originally solidified, and separating
them from the banded areas of eutectic. This is particularly
well seen in alloys of copper with cuprous phosphide, and
of copper with silver, containing copper in excess of the
eutectic proportion.' The effect may be due to the continued

' E. Heyn, Zeilsch. anorg. Chem., 1904, 39, I.
^ Giraud, Rev. de Mitallurgie, 1905, 2, 297.

' A. K. Huntington and C. H. Desch, Trans. Faraday Soc, 1908,
4, 51-

UNDERCOOLING AND METASTABLE STATE 205

separation of copper after passing the eutectic point, crys-
tallization of the excess of the second constituent then
taking place suddenly, forming an envelope, after which
the eutectic composition is again reached, and simultaneous
separation of the two constituents takes place in the normal
manner. The fact, however, that the effect is only seen when
the primary crystals are well-orientated skeletons, and never, in
the author's experience, when isolated crystals are produced,'

neo-

/

lltH)

Liquid /

1120

/

IIOO

/ + liguicU

loaa

+ tt«7iti2r~--~.._/ 7065°

1060

1 ~~~

/ CVy + CUz

OIZ3f-5GTQ9 JOVa

Cvbf O hyweighib

Fig. 76. — Copper and cuprous oxide.

suggests a different explanation, which is that proposed by
Huntington and Desch, namely, that the envelope is due to
segregation at the moment of solidification of the eutectic.
The directive force of the copper crystals is great, and it would
seem probable that when solidification of the eutectic begins,
the copper particles are attached to the previously formed
skeletons, thrusting aside the particles of phosphide, or of

* A similar band is seen surrounding the skeletons of iron in iron-
phosphorus alloys, but not in those richer in phosphorus, in which the
primary crystals are isolated rhombs of FejP (J. E. Stead, J. Iron Steel
Inst., 1900, ii., 60). Segregated bands also surround the crystals of copper
in alloys of copper and silver, as is seen in Plate III., A.

2o6 METALLOGRAPHY

silver, etc., which have a less developed power of orientation.
This process would continue until a layer of the second con-
stituent had been formed, completely coating the skeletons,
and the eutectic would then be free to assume the banded
structure, with a general parallelism determined by the previous
orientation. To decide the point, it remains to be determined
whether the formation of the layer is inhibited by inoculation
with the second constituent, so preventing undercooling.

Undercooling may also take place when the constituent
crystallizing is a solid solution. In the only case which has
yet been studied experimentally, that of mixtures of naphtha-
lene and /3-naphthol, the liquidus and solidus curves lie very
close together, and the supersolubility curve lies about 3°
below the latter, and parallel with it.' It was not found
possible to determine the exact composition of the crystals
separating from the undercooled solution. It would be of
great interest to examine a case in which the liquidus and
solidus curves are separated by a wide interval, as in the alloys
of gold and platinum, and to determine whether the composi-
tion of the solid separating is given by the intersection of the
solidus with the horizontal drawn through the point at which
crystallization actually commences, or through the correspond-
ing temperature on the liquidus, that is, the temperature to
which the mixture rises as soon as crystallization in the meta-
stable region has begun. The answer to this question is at
present uncertain, although the microscopic appearance of
such alloys would seem to render the former suggestion the
more probable.

Should the chemical changes taking place during freezing
be of a kind to require some time for their accomplishment,
the occurrence of undercooling may result in an entire alteration
of the form of the freezing-point curve. Such a case has been
observed in the alloys of antimony and cadmium.^ In the

' H. A. Miers and F. Isaac, Trans. Chem. Soc, 1908, 93, 927.

^ N. S. Kumakoff and N. S. Konstantinoff, Zdtsch. anorg. Chem.,
1908, 58, I. Compare W. Treitschke, ibid., 1906, 60, 218. The diagram
is given by both observers in an incomplete form ; it has been completed
from their thermal and microscopical data.

UNDERCOOLING AND' METASTABLE STATE i&j

diagram, Fig. 77, the full curve corresponds with the condition
of stable equilibrium, and is only obtained after inoculation
with crystals of the compound CdSb, the dotted curve repre-
senting a metastable condition realized in alloys cooled mthout
stirring or inoculation. The stable curve has a maximum at
50 atomic per cent. Sb, with a eutectic point, CdSb — Sb, at 58'4
atomic per cent. Sb. In the absence of inoculation, however,
the curve corresponding with the separation of free antimony is

°C
6S0

soo-

70 80

4-50

c

300

2SO

Liguidy

/^ Sib
/ -fc Zi^qxvpd.

yiUQuM

St> + ligitMl

y/cd3 Sb2

y / + liguid

CdjSb^

+
Cii,£b

Cd Sh + Sb
Cd^Sbz+ Sb

Cd,+ Cd.^ Sbz

so /oo%sh

20 30 4-0 SO eO 70 80 90 lOO

Ato7tv%Sb

o to

Fig. 77. — Stable and metastable systems in alloys of antimony and cadmium.

prolonged downwards, and when separation of a second solid
phase does take place, it is not the stable compound, CdSb,
that crystallizes, but the metastable compound, CdsSba, which
has its own solubility curve, with a maximum at 40 atomic per
cent. Sb and 423°, 32° lower than the maximum on the stable
curve. This is in accordance with the general rule that meta-
stable modifications melt (or freeze) at a lower temperature
than stable modifications.

-Consider now an alloy containing 53 atom, per cent. Sb.

2o8 METALLOGRAPHY

If slowly cooled and inoculated, it deposits crystals of CdSb,
and shows a eutectic point at 445°. If cooled without inocu-
lation, crystallization does not set in until later, when antimony
separates, and there is a eutectic arrest at 402°, corresponding
with the simultaneous separation of Sb and CdaSba. On further
cooling, the reaction

CdaSba + Sb = sCdSb

occurs with considerable development of heat, and the solid
alloy passes, more or less completely, into the stable condition.
To the left of the vertical line at 50 atom, per cent. Sb, the
compound CdjSbj is stable at low temperatures. The develop-
ment of heat is observed in metastable alloys containing
between 40 and 50 atom, per cent. Sb, owing to the transfor-
mation of their excess CdsSbg. To the left of the line at
40 atom, per cent. Sb, again, Cd3Sb2 is alone stable.

A similar effect is observed in alloys of zinc and antimony ; '
but in this case it is the compound ZugSba which occurs at the
maximum, and it is the compound ZnSb which is not deposited,
unless crystals of it are used for inoculation.

Again, reactions which take place between the crystals
which have separated at a higher temperature and the still
liquid alloy, and are indicated by a break in the direction
of the freezing-point curve (p. 36), are particularly liable to
undercooling, as one of the reacting substances is solid, so that
the velocity of the reaction is limited by the solution and
re-deposition of the solid phase, or by the velocity of diffusion
in the solid solution. Both, of these processes are slow in
comparison with that of crystallization. Very slow cooling
and thorough agitation are therefore necessary if the reaction
is to proceed to completion.

The possibility of undercooling is not confined to the
passage from the liquid to the solid state, but is also associated
with the physical and chemical changes taking place after
solidification. A typical example is that of a solid solution
breaking up into two constituents when cooled to a certain
transformation temperature. It has been mentioned on p. 59
' S. F. Schemtschuschny, Zeiisch. anorg. Chem., igo6, 49, 384.

UNDERCOOLING AND METASTABLE STATE 209

that iron containing o'Sg per cent, of carbon is, at temperatures
above 700°, a homogeneous solid solution. At 690° this
solution resolves itself into a conglomerate, or eutectoid, of
iron and iron carbide, FejC, the process of resolution being
completed at constant temperature under conditions favouring
equilibrium. In the ordinary cooling of a bar of steel, as an
iron-carbon alloy of such a composition is called, the tempera-
ture falls below that at which the change should take place,
then rises to 690°, and continues at that temperature, the
course of events bearing a complete resemblance to that
already described in the freezing of an undercooled liquid.
The amount of heat developed by the transformation is large,
and when a bar of steel, of the stated composition, is cooled in
air, the rise of temperature due to the change is sufficient to
cause a visible brightening of the red glow emitted by the
metal, if observed in a darkened room. The phenomenon,
which was first observed by Barrett,^ is due to the breaking up
of the solid solution, martensite, into a mechanical mixture of
pure iron (ferrite) and iron carbide (cementite). It has received
the name of " recalescence," and the term has since been
applied more generally to those developments of heat which
occur during the cooling of many solid alloys. Very marked
instances are observed in the ternary alloys used as fusible
metals, which often show a sudden rapid rise of temperature
during cooling, accompanied by a change of volume sufficient
to cause the alloy to fly to pieces.^

The process of " quenching," so often applied to alloys
both in metallographic investigations and in technical
practice, consists in cooling a solid alloy through a critical
range with such rapidity that the transformation which takes
place on slow cooling is either entirely or partly suppressed,

' W. F. Barrett, Phil. Mag., 1873, [iv.] 46, 472. The momentary
elongation of a stretched steel wire on cooling to this temperature was
observed by G. Gore, Proc. Roy. Soc, 1869, 17, 260, and it was while
investigating this volume change that Barrett observed the glow. The
effect is easily shown as a lecture experiment.

" Erman, Ann. Physik, 1827, [ii.] 9, 557; R. Warington, Mem.
Chem. Soc. Land., 1843, 1, 77 ; Person, Compt. rend., 1847, 25, 444 ; W.
Spring, Ann. Chim. Pliys., 1876, [v.] 7, 178.

T.P.C. P

2 lo METALLOGRAPHY

so that a phase which is stable only at some higher temperature
is retained in the cooled alloy in a metastable or labile con-
dition. To take an instance of the technical application first,
the well-known hardening of steel by quenching depends on the
suppression of the recalescence point by rapid cooling through
the critical range. The solid solution of carbon in iron, which
exists at high temperatures, is thus prevented from breaking up
into free iron and the carbide, and so forming a conglomerate
of greatly inferior hardness. Many copper alloys, especially
those containing certain proportions of zinc, tin, or aluminium,
may also be hardened by quenching, although not to so great
an extent as steel. The mechanism of the process is in each
case the same — the preservation by rapid cooling of a solid
solution which is normally broken up in slow cooling before
reaching the ordinary temperature.

Quenching is resorted to in metallographic researches as a
means of investigating the nature of the phases existing at high
temperatures. The examination of such phases at the tempe-
rature at which they are stable is in general impracticable,
although the crystalline structure of iron at a red heat has been
studied by etching with fused calcium chloride,' the etching
figures thus produced being visible after cooling. This method
is perhaps applicable in other cases, although the liability of
heated metallic surfaces to become covered by a film of oxide
is a source of difficulty. Apart from this treatment, the only
means of obtaining information as to the microscopic struc-
ture of phases unstable at the ordinary temperature is that of
quenching. Further, the physical and mechanical study of such
undercooled phases and supersaturated solid solutions is one
of great interest, and it is therefore desirable that the quenching
process adopted should be applicable to specimens large
enough for the purposes of physical and mechanical tests.
The systematic investigation of alloys by quenching is explained
in Chapter XIV.

In technical and ordinary laboratory practice, the metal or
alloy is heated to the required temperature in a muffle or
electric furnace, seized with heated tongs, and rapidly plunged

' p. H. Saniter, y. Iron Steel Inst., 1897, li. 115 ; 1898, i. 206, 275.

UNDERCOOLING AND METASTABLE STATE 211

into cold water. As a method of research, such a procedure is
quite insufficient. Cooling to an unknown and inestimable
extent takes place during the opening of the furnace and the
transfer of the specimen, so that the actual temperature of
quenching is quite unknown. Better results are obtained by
heating the specimen in a bath of fused salts, such as the alkali
chlorides, borates, or silicates, or mixtures of these. Harden-
ing furnaces of this type are now in use for technical purposes.^
On removing the specimen, a film of salt, of low conducting
power, remains adhefent to the metal, and hinders the fall of
temperature during the transfer to the cooling bath. This plan
has been adopted by Le Chatelier in his researches on the
quenching of tool steel.^ It is, however, desirable that the
transfer should be more rapid than is possible by such means.
Benedicks, in a very thorough investigation of the quenching
process,' used a horizontal electric furnace of special construc-
tion, having a longitudinal slot in the under side for one-half
of its length. The specimen, with a thermo-couple attached,
was mounted on a pivoted arm. During heating it occupied
the centre of the furnace, and when the quenching temperature
was reached, the arm was released electrically and swung by a
spring through a quarter-circle, passing through the slot and
carrying the specimen downwards into the quenching vessel.
This arrangement provides for a very rapid transfer of the
specimen.

In ordinary practice small specimens may be quenched
satisfactorily by employing a tube extending beyond the limits
of the horizontal furnace, and provided with a T-piece directed
downwards. The object is rapidly drawn by a platinum wire,
or pushed by a quartz or porcelain rod along the tube, until it
falls into the quenching liquid through the T-piece.* As the
tube may be closed with stoppers at both ends, the wire
passing through a fine perforation, it is possible to work in an

' E. Sabersky and E. Adier, Trans. Faraday Soc, 1909, 5, 15.

* Rev. de Miiallurgie, 1904, 1, 184.
» y. Iron Steel Inst., 1908, ii. 152.

* This device, without the addition of the T-piece, was employed in
quenching specimens of cast-iron by P. Goerens, Vorgange bei der Erstar-
rung und Vmwandlung von Eisenkohlenstoffkgierungen, Halle, 1907.

212 METALLOGRAPHY

indifferent atmosphere, which is advantageous in experiments
with steel, the surface of which is liable to become oxidized or
decarburized.

Instead of removing the specimen from the furnace and
transferring it to the cooling bath, we may admit the quench-
ing liquid to the heated space. This method, which has only
been rendered possible by the advent of fused silica as a
common laboratory material, has been adopted by Rosenhain,'
whose apparatus is illustrated diagrammatically in Fig. 78. A
wide silica tube passes through a horizontal resistance furnace,
and is fitted at one end with a large tap. A, and an arm bent at
right angles. The other end is connected with a suction flask

^^8=^

1

Thermo
couple

Fig. 78. — Rosenhain's quenching apparatus.

by means of a T-piece, through which the thermo-couple
passes. The specimen, previously polished if necessary, is
placed in the silica tube, and the stopcock is closed. The bent
arm dips into a vessel of water. When heating begins, the
flask and tube are exhausted by means of a Fleuss pump.
When the pyrometer indicates that the desired temperature
has been reached, the stopcock A is suddenly opened, when
water rushes in to fill the exhausted space. The silica tube
withstands the sudden cooling, and the specimen is driven by
the inrush of water to the further end of the tube, where it is
cooled very rapidly. Quenching takes place quietly, without
explosive violence, and all exposure to the atmosphere is
avoided, so that the surface of the alloy is perfectly protected
from oxidation.

' J. Iron Steel Insl., 1908, i. 87.

UNDERCOOLING AND METASTABLE STATE 213

The taking of a cooling curve during quenching, although
a matter of some experimental difficulty^ is necessary if it is
desired to know whether the recalescence has been entirely
suppressed or not. The problem has been attacked by Le
Chatelier,^ and also by Benedicks {loc. cit.). The thermo-
couple is enclosed in a thin iron tube, being held in a fixed
position inside it by a plug of clay, introduced while moist
and subsequently baked. The tube fits tightly into a hole
bored longitudinally in the specimen, and the welded junction
of the couple is in close contact with the latter. The entry of
water at the joint is prevented by a coat of sodium silicate.

The galvanometer employed by Le Chatelier was insuffi-
ciently sensitive, and a great improvement was introduced by
Benedicks, who used a sensitive string galvanometer of high
resistance. In this form of instrument^ a silvered quartz
fibre, through which the current to be measured passes, is
placed in a strong magnetic field, and its lateral displacement
is made visible by means of a mirror. The magnified move-
ments are recorded photographically. The resistance of such
an instrument is so high that its readings are quite independent
of changes in the resistance of the thermo-couple, and it
responds with great rapidity to changes of temperature.

Both authors have compared the cooling effect of different
liquids. The statement is often made in the older text-books,
and is commonly believed by practical men, that the thermal
conductivity determines the cooling power of the liquid, and
various salts, etc., are added to water in tool- hardening shops
for the purpose of increasing the conductivity. Exact experi-
ment shows that this is a mistaken opinion, the cooling due to
liquid conduction being, as might be expected, quite insignifi-
cant in comparison with that due to convection. The true
determining factor is the specific heat of the liquid. Mercury,
which is popularly supposed to have the greatest quenching
power, is much less effectual than water. The addition of
salts or acids to water has very little effect on the quenching
power. The larger the quantity of liquid employed, relatively

' Rev. de MUallurgk, 1904, 1, 473.

' M. Edelmann, jun,, Physikal. Zeitsch., 1906, 7, 115.

214 METALLOGRAPHY

to the mass of metal or alloy, the more rapid the cooling.
The most effectual quenching agents examined are the alcohols,
and these may be expected to give the most complete suppres-
sion of the recalescence in experiments of the kind. Liquids
at or near their boiling-point have a greatly reduced quenching
power.

A liquid of low quenching power may be employed delibe-
rately when it is desired to hinder complete transformation
without entirely suppressing it. An instance of this is seen in
the treatment of steel forgings for the manufacture of large
guns. The forgings are heated to 920°, and are then rapidly
lowered, in a vertical position, into a tank containing rape oil,
a liquid of low cooling power, the tank being often contained
in a water-jacket. Pumps may be used to produce a circula-
tion of the oil, and so render the cooling of the mass uniform.
The steel passes rapidly through the critical range, and the
solid solution, stable at 920°, breaks up too rapidly to allow of
a mechanical separation of the iron and carbide, such as takes
place in slow cooling. The forgings are afterwards annealed,
to remove strains produced by quenching.

The partial resolution of the solid solution in slowly
quenched iron-carbon alloys results in the formation of very
intimate mechanical mixtures of the constituents, apparently
homogeneous in some cases, which have often been regarded
as distinct phases, and have thereby introduced some confusion
into the study of hardened steels. The iron-carbon equilibrium
is discussed in Chapter XVII.

The phases retained by quenching are commonly spoken of
as metastable, but, although experimental proof has not been
produced, we are bound to assume that the metastable limit
has been passed, and that the systems are correctly described
as labile, and not as metastable. Quenched specimens there-
fore tend spontaneously to assume an equilibrium condition
by undergoing the transformation which was suppressed by
quenching. It is uncertain how far the spontaneous change
can proceed at ordinary temperatures. White tin, which is
only stable above 18°, does not change into grey tin at 0°
unless brought into contact with the new phase. Below 0°

UNDERCOOLING AND METASTABLE STATE 215

the change may occur spontaneously, although even at low
temperatures it may fail to occur when the tin is kept for long
periods.^ The maximum velocity of transformation is near
-45°.

Hardened steel is undoubtedly labile at the ordinary tempe-
rature; the presence of the stable phase is not necessary to
initiate change. But there is little evidence that softening
of the steel takes place spontaneously unless the temperature
is raised. Japanese swords of the fifteenth century, if care-
fully preserved, are found to be as hard at the edge as if newly
hardened ; it would therefore seem that no appreciable return
to the stable state takes place in the course of several centuries
at atmospheric temperatures. On the other hand, prolonged
heating of hardened steel even to 1 00° produces an appreciable
softening, and heating to 150°, in a few minutes. The whole
question of labile and metastable conditions in undercooled
solid alloys demands fuller investigation. Certain conditions
of physical, as distinguished from chemical, metastability are
discussed in Chap. XVI.

' E. Cohen and C. van Eyck, Zeitsch. physikah Chein., 1899, 80, 601 j
E. Cohen, ibid., 1900, 33, 59 ; 35, 588 ; 1901, 36, 513.

CHAPTER XI

DIFFUSION IN THE SOLID STATE

The possibility of diffusion in the solid state has already been
assumed in dealing with equilibrium in the crystallization of
solid solutions and the reaction of a solid and liquid phase to
form a compound. Many facts known tc the technical metal-
lurgist, and many isolated laboratory observations, point to the
existence of such a process, but the earliest systematic and
quantitative investigation bearing on the question is that
described in the Bakerian lecture of Roberts-Austen for 1896.^
An example taken from this research will indicate the nature of
the process.

An alloy of lead and gold containing 5 per cent. Au is
pressed tightly against one end of a cylinder of pure lead, the
two surfaces in contact being accurately plane, and the cylinder
is then heated at 165° for thirty days, after which it is sawn
into sections, and the gold is estimated in each section by the
ordinary methods of assaying. Such a cylinder, 0-64 cm. high,
f)laced vertically with the gold alloy downwards, gave the
following figures on assaying : —

No. of section from
base.

Weight of section.

Per cent. Au.

Diffusivity,

I
2
3

grams.
0-64

2-33

2-02

0-039
0-030
o'oi5

o'oos

• W. C. Roberts-Austen, Phil. Trans., 1896, 187a, 383. See, for a
summary and bibliography of the subject of diffusion in solids and glasses,
C. H. Desch, Brit. Assoc. Ref., 1912.

216

DIFFUSION IN THE SOLID STATE 217

The number in the last column represents the difFusivity,
expressed in sq. cm. per day, calculated by means of Pick's
theory of diffusion (see Chapter XV.). The diffusion is still
measurable at so low a temperature as 100°. At higher tempe-
ratures the process is much more rapid, so that gold is able to
rise against gravity to a height of 7 cm. in lead at 250° in less
than a month. The latter temperature, although 77" below
the melting-point of lead, is above the eutectic temperature of
the lead-gold series, and the possibility of the metal being
transported in the form of a liquid eutectic alloy is therefore
suggested; but the concentration of the gold is always far
below the eutectic proportion, and in all probability below the
limiting concentration of the solid solution of gold in lead.
The following values were found for the diffusivity at different
temperatures ; —

251° o'o23, o'o30

200° o"oo7i o'ooS

165° o'ooS, o'oo4

100° O'OOOOZ, 0'00002

The fact that smooth surfaces of lead and gold, pressed
together in vacuo at 40° for four days, adhere so firmly that a
force equal to one-third of the breaking stress of lead is
required to separate them, seems to indicate that diffusion also
takes place at the ordinary temperature.

Gold diffuses in silver at 800°, the diffusivity being of the
same order as that of gold in lead at 200°. It had been shown
in 1894^ that carefully surfaced cylinders of copper and zinc,
heated 6 or 8 hours at 400°, formed a layer of yellow alloy to
a depth of o*8 mm., and there is much evidence to show that
the phenomenon is a very general one.

The diffusion of a substance into the interior of a solid
metal assumes great importance in the technical conversion of
iron into steel by "cementation." The iron is heated in
powdered carbon, and diffusion takes place, causing carburiza-
tion of the iron to a depth depending on the temperature and
on the duration of the heating. A quantitative study shows
' W. Spring, Bull. Acad. roy. Belg., 1894, [iii.] 28, 23.

2i8 METALLOGRAPHY

that the carburization is a process of true diffusion, and the
curve representing the distribution of carbon at different depths
has the form of a diffusion curve.^ The process has been
explained as one of penetration of the iron by gases derived
from the carbon, since gases, especially those containing
nitrogen, undoubtedly have a great influence in certain forms
of the technical process.^ It is unnecessary, however, to assume
that the gas passes into the metal as such, and there is much
evidence that its effect is rather to produce a layer of highly
carburized alloy on the surface, which forms the starting-point
of the true solid diffusion, like the rich lead-gold alloy used in
Roberts-Austen's experiments.

Much controversy has turned on the carburization of irori
by diamond in vacuo, at a temperature much below the
melting-point of the iron. The experiment was at first held
to be conclusive,' but later experiments have shown that traces
of gas have a great influence in promoting cementation,' whilst
the actual contact of the specimens is also of great importance
in ensuring a positive result.^ The most conclusive experi-
ments are those of F. Weyl,^ who showed that cementation
took place in a high vacuum if contact were maintained, but
that the presence of a very thiin intervening layer prevented
any action. It is certain that the carbide first forms a solid
solution with the iron, and that diffusion then takes place
within the solid solution from places of higher to those of
lower concentration, until a limit of saturation is reached. In
accordance with this statement, carbon is found to diffuse, in
the form of carbide, from a high-carbon steel into a specimen
•of soft iron placed in immediate contact with it.'

' R. Mannesman!!, Verh. Ver. BefSrd. Gewerbefl., 1879, 58, 31 ;
W. C. Roberts-Austen, J. Iron Steel Ittst., 1896, i. 139.

* For an account of the action of gaseous oxides of carbon as cementing
agents, see F. Giolitti, J. Iron Steel Inst., 191 1, ii. 307 ; where previous
ipapers by the same author are summarized.

' W. C. Roberts- Austen, Nature, 1889, 41, 14; F. Osmond, Compl.
^rend., 1891, 113, 578.

* G. Charpy and S. Bonnerot, Compt. rend., 19 10, 150, 173.
» L. Guillet and C. Griffiths, ibid., 1909, 149, 125.

" Melallurgie, 1910, 7, 440.

» J. U. Arnold and A. Mc William, J. Iron Steel Inst., 1899, i. 8j.

DIFFUSION IN THE SOLID STATE 2iq

The earliest scientific record of the production of an alloy
other than steel by a process resembling cementation is the
observation by Faraday ^ that wires of steel and platinum could
be welded in a bundle to form a rod, which on testing with
acid proved to be an alloy. As the metals of the iron and
platinum groups are known to form solid solutions with one
another, there is no doubt that the welding depends on true
reciprocal diffusion of iron and platinum. Many alloys have
also been produced by bringing the two metals into intimate
contact by the application of pressure, and heating the com-
pressed mass to a temperature below the melting-point of the
most fusible alloy of the series," although the production of
fusible alloys by this process is not evidence of diffusion,
as an intimate mixture of fine particles will melt at or near
the eutectic temperature (see p. 29). Microscopical ex-
amination shows that union takes place particularly between
those metals which form inter-metallic compounds, but that
the typical eutectic and other structures characteristic of
solidified metals are not produced. The fact of union is
most patent when metals which form coloured compounds,
such as copper and zinc or copper and antimony, are heated
together.

The superficial conversion of a metal into an alloy by
surrounding the specimen with a metallic powder and heating
to a temperature favourable to diffusion has received technical
application in the coating of iron and other metals with zinc'
The pieces of metal to be coated are packed loosely in zinc
dust which contains sufficient zinc oxide to prevent caking,
and are heated for several hours at 250-300°, the furnace
being rotated to bring fresh material into contact with the
surface. A microscopic section normal to the surface shows

' M. Faraday and Stodart, Quart. J. Sci., 1820, 9, 319 ; reprinted
in Faraday's Experimental Researches in Chemistry and Physics, 1859,

P- .57.

' W. Spring, Ber., 1882, 15, S9S 5 Zeilsch. physikal. Chem., 1888, 2,,
532. 536; Bidl. Soc. chim., 1888, [ii.] 49, 215 ; Bull. Acad. roy. Belg.,
1883, [iii.] 6, 229, 492 ; W. Hallock, Amer. J. Sci. 1889, [iii.] 37, 402 j
Zeitsch. phxsical. Chem., 1888, 2, 378.

» S. Cowper-Coles, Electrochem. and Metall., 1904, 3, 828.

220 METALLOGRAPHY

that the iron and zinc are united by a layer of zinc-iron alloy,
although the temperature is much below the melting-point of
zinc. A similar result is obtained when copper is used instead
of iron, an outer layer of an alloy rich in zinc being formed,
united to the copper by a layer containing a smaller proportion
of zinc. Antimony, tin, or aluminium may replace zinc, the
powdered metal being used either alone or mixed with its
oxide or some inert matter to prevent caking.

It is possible that zinc vapour, which is given qff freely
below the melting-point of zinc, may be instrumental in the
formation of the first coating. The subsequent production of
a layer of alloy of the two metals is, as will be shown later, a
process of true solid diffusion.

A few more instances of the penetration of solids into solids
by diffusion may be mentioned. Sulphur diffuses into silver,
blackening it to a considerable depth,^ and even iron has been
observed to penetrate into silver in the course of some years.'^
Both iron and platinum, heated in a mixture of carbon and
silica, are converted into silicides.^ Porcelain heated in
amorphous carbon or graphite at 1000-1500°, is blackened,
and the material in the interior of the mass is proved
chemically and microscopically to be pure carbon.^

A very remarkable instance is observed in the electrolysis
of glass. If a cell, divided into two parts by a glass septum,
is filled on one side with pure mercury and on the other with
sodium amalgam, and a current passed tending to drive the
sodium towards the mercury, the temperature being near 200°,
electrolysis occurs, and sodium appears in the mercury on the
further side of the septum, ° the glass remaining clear and

' Homberg, Mem. Acad. Roy. Set., 1713, 306 (published 1739).
Homberg also observed that a complex mixture, consisting probably
chiefly of a silver amalgam, passed through a plate of silver without
rendering it brittle.

^ J. H. Howell, Nature, 1906, 73, 464.

' A. Colson, Compt. rend. , 1882, 94, 26.

* R. S. Marsden, Proc. Roy. Soc. Edin., 1880, 10, 712 ; J. Violle,
Compt. rend., 1882, 94, 28.

' E. Warburg and F. Tegetmeier, Ann. Physik., 1890, [iii.] 41, i j
repeated and confirmed by W. C. Roberts-Austen, Proc. Inst. Mech. Eng.,
189s, 238.

A. Diffusion of zinc into copper X 30.

B. Diffusion of zinc into copper. X 30.
PLATE X.

[ To face page 221.

DIFFUSION IN THE SOLID STATE 221

unchanged. If lithium amalgam is substituted for sodium
amalgam, sodium passes into the mercury as before, and
lithium enters the glass in its place, the glass becoming
opaque. When the opacity extends completely through the
glass, lithium begins to appear in the mercury. If, however,
potassium is used instead of lithium, the .expulsion of sodium
does not take place. The atomic volume of lithium is less,
and that of potassium greater than that of sodium, and it
appears therefore that a metal of small atomic volume is able
to follow in the path of one of larger atomic volume, as if, as
Roberts-Austen express it, actual galleries were formed in the
glass by the sodium atoms, which could be traversed by smaller,
but not by larger, atoms of a different element. This remark-
able fact points to the importance of the atomic volume in the
study of the characteristic behaviour of solids.

The process of diffusion may be most conveniently followed
in a system in which several solid phases, distinguishable by
the microscope, may be formed. Such a system is found in the
alloys of copper and zinc. On immersing a rod of copper for
a short time in molten zinc, an adherent coating is formed,
consisting of a brittle silvery-white alloy rich in zinc. A
section cut through the rod shows that within this layer is a
layer of a yellow alloy, the )8 solid solution, which appears,
under a low magnification, to be separated from the unaltered
copper of the interior of the rod by a sharp boundary. This
structure is shown in the photo-micrograph, Plate X., A, in which
the upper band represents the outer layer, which consists, at
least near its contact with the inner layer, of the 7 solid solution.
Next to it, and sharply divided from it, is a band of the /3
phase, and next to it again, separated by a very narrow dark
band, is the core of unchanged copper.

If the rod is now heated for several hours at a temperature
of about 700°, and then slowly cooled, diffusion is found to
have occurred when the specimen is again examined, zinc
diffusing inwards from regions of high to those of low con-
centration, that is, from the outer zone to the inner core. The
y zone gradually disappears, whilst the ^ zone undergoes a great
extension of breadth, and also loses its original homogeneity,

222 METALLOGRAPHY

showing isolated y crystals near its outer margin and a crystals
towards the inner margin, the latter becoming more and more
closely grouped until a zone of the a solid solution is formed.
This constituent being isomorphous with copper, it passes in-
sensibly into the central mass, the transitional zone exhibiting a
continuous change of colour from yellow to red. After further
heating, the y crystals disappear, and a broad band of /? con-
taining numerous u. crystals forms the outer layer. This con-
dition is seen in Plate X., B, where the upper band consists of
a -|- y8, the intermediate zone of pure a, and the lowest of copper.
If the experiment is repeated, using other metals in place of
zinc, the outer layer is frequently found to break off before
diffusion has proceeded very far, owing to the large increase
of volume due to combination.

A conglomerate composed of particles of bismuth and
thallium, rendered compact by pressure, proves convenient
for the study of diffusion, owing to the' formation of two
coloured layers, one of which contains the compound BijTlj.'
The diffusion may be detected by means of the microscope
after a year at atmospheric temperature.

The most recent, and in some respects the most satisfac-
tory, experiments are those of G. Bruni and D. Meneghini.^
These authors employed metals of high melting-point, which
form solid solutions without chemical combination, and they
followed the progress of the diffusion by means of the change
in the electrical conductivity. A nickel wire, o"5 mm. di-
ameter, was coated electrolytically with copper until the
increase of weight corresponded with 59 per cent, copper and
41 per cent, nickel. The compound wire was then heated to
1000° in hydrogen, and the conductivity was determined from
time to time. A slight increase of conductivity was noticed in
the first hour, owing to the conversion of electrolytic copper
into the better-conducting annealed form, but after this a con-
tinuous decrease of conductivity was observed, the minimum
being practically reached after 140 hours, when the value

• G. Masing, Zdtsch. anorg. Chem., 1909, 68, 265.
'' Atti R. Accad. Lined, 191 1, [v.] 20, i. 671, 927; Intern. Zeitsch.
Metallographie, 1912, 3, 26.

DIFFUSION IN THE SOLID STATE 225

attained was exactly that of a homogeneous alloy of the same
composition. Exactly similar results were obtained when gold
wires were coated with silver or copper. In a further experi-
ment the area of contact of the two metals was greatly increased
by coating a copper wire, 0-075 "i"^- diameter, with nickel and
copper alternately, until thirty layers of each metal had been
deposited. The thickness of the layers gradually diminished
towards the outside, ultimately reaching a value of about 2 -5 /*.
Such a wire became homogeneous, as shown by microscopical
examination as well as by the determination of the con-
ductivity, after less than two hours at 1000°, whilst diffusion
proceeded at an appreciable rate at 500°.

Segregation in Solid Alloys

The process of diffusion in solid alloys is further illustrated
by the change known as segregation. If one of the solid phases
present in the alloy is distributed in the form of minute crystals
or crystallites, prolonged heating at a temperature below that
at which any liquid is formed frequently results in the dis-
appearance of many of the scattered particles, and in the
formation of larger masses or crystals of the same constituent
An instance of this is the behaviour of iron-carbon alloys when
heated below the critical point Ari. Pearlite, which is the
finely laminar eutectoid of iron and cementite, gradually loses
its laminated character between 600° and 670,° the cementite
" segregating " to form granular masses.' If the steel contains
only about 5 per cent, of pearlite, the remainder being ferrite,
the whole of the eutectoid may be broken up, so that the steel
finally consists only of ferrite, containing a few isolated masses
of cementite. Hence the apparent contradiction of the presence
of structurally free ferrite and cementite in the same specimen,"
the fact being that only the ferrite is a primary constituent, the
eutectoid which originally accompanied it having disappeared
through the segregation of its component particles. If the steel
consists of pure pearlite, segregation does not proceed to the

' J. E. Stead, J. Soc. Ckem. Ind., 1903, 23, 340.
^ E. F. Lange, Metallografhist, 1903, 6, 9,

224 METALLOGRAPHY

same extent, and the change ceases when the lamellae have
been replaced by granules of cementite. On the other hand,
a steel containing an excess of cementite may undergo marked
segregation, the whole of the pearlite being broken up, its com-
ponent cementite uniting itself to the original cementite bands,
and the ferrite forming broad envelopes around these.

The iron-iron phosphide eutectic behaves in the same way,
the ferrite, containing 170 per cent, of phosphorus in solution,
and the phosphide, FesP, segregating to form broad bands.'
Other eutectics, such as that composed of copper and copper
phosphide, may also be rendered much coarser in structure by
heating below the eutectic point.

The cause of segregation must be sought in the instability
of minute crystals of a substance in presence of larger crystals
of the same kind and of a solvent. The solubility of minute
crystals of a salt in water is greater than that of larger crystals
of the same salt. For example, calcium sulphate, CaSO^,
2H2O, in particles not less than 2 /a (/t = o'ooi mm.) in diameter,
has a solubility in water at 25° of 2 '085 g. per litre, whilst
particles 6nly o'3 ft in diameter have a solubility of 2 '476 g.
per litre. Barium sulphate dissolves to the extent of 2-29
milligrams per litre when the diameter is i'8 /t, but of 4- 15
milligrams per litre when it is o'l ju.." Consequently, when
small and large crystals of a salt are placed in a saturated
solution of the salt in water, the smallest crystals dissolve.
The solution then becomes supersaturated with respect to the
larger crystals, which therefore increase in size, and continue
to grow at the expense of the small crystals until the latter
have been consumed. The same process occurs in solids. The
larger crystals of the solid phase dispersed through the alloy
grow at the expense of the smaller. The possibility of such
a process presupposes a certain solubility of the particles in the
intervening substance, and we must therefore assume, in the
case ,of the iron-carbon alloys, a certain solubility of cementite
in iron even below the critical point Arj. This is in accor-
dance with the experimental results of Benedicks and others.

' Stead, loc. cit.

' G. A. Hulett, Zeitsch. physikal. Chem., igoi, 37, 385.

DIFFUSION IN THE SOLID STATE 225,

The dispersion of a newly formed solid phase in an ultra-
microscopic form through a crystalline mass of metal is not
uncommon.' It is observed in alloys of nickel and iron, of
cadmium and tin,^ and also in the j8 solid solution of copper
and zinc' This last case is of special interest. The )8
solution is not stable below 470°, at which temperature it is
resolved into a mixture of the a and y phases, which at first
remain in a state of ultramicroscopic division. Segregation
is extremely slow, even at temperatures only slightly below the
transformation point, but is accelerated by the presence of the
a or the y phase in excess, or by the presence of a third metal
in solid solution, such as aluminium.

The transfer of matter from the smaller to the larger
particles can only take, place by diffusion in solid solution, of
which such segregation is a further instance. In reality,
diffusion must take place whenever crystals are deposited from
a solid solution. Just as, in the growth of crystals in a
liquid, material for growth must be supplied from the sur-
rounding solution by diffusion as that in the neighbourhood of
the growing crystal is consumed, so the supply in a solid
solution must take place in the same way. Convection cur-
rents, which also play a part in the crystallization from liquids,
are of course excluded in the case of solids.

Annealing
The operation of annealing, performed on metals and
alloys, consists in subjecting them to continued heating at a
suitable temperature, below that at which any liquid phase is
formed. If the specimen has previously been subjected to
any kind of mechanical deformation by hammering, rolling,
drawing, etc., the first effect of annealing is to remove the
strains thereby produced.. This effect will be discussed in
Chapter XVI., in connection with the study of deformation.
Strains may exist even in cast metals, owing to unequal
velocities of cooling at different parts of the specimen, causing
contraction to take place unequally.

' C, Benedicks, Zeitsch. Chem. Ind. KoUoide, 1910, 7, 290.

= W. Guertler, Intern. Zeitsch. Melallographie, 191 2, 2, 172.

» H. C. H. Carpenter, Jour. Inst. Metals, 1912, 7, 123.

T.P.C. Q

226 METALLOGRAPHY

Leaving aside the relief of mechanical strain, the effects of
annealing are : to alter the size of the crystal grains, and to
bring alloys to a condition of equilibrium by facilitating
diffusion in the solid state.

It is well known that a metal or alloy may be rendered
coarser in grain by prolonged heating at a sufi&ciently high tempe-
rature. Thus the size of grain of almost pure iron increases
with the temperature. Recrystallization proceeds rapidly at
730°, so that after twelve hours the individual grains are visible
without magnification.^

The growth of crystals in lead may be observed even at
60°, if the metal has been previously strained.^ The increase of
coarseness brought about by the annealing of solid solutions, such
as the a solution of ordinary brass, is very pronounced.^ The
process of recrystallization may be watched by re-etching lightly
a marked area of the specimen after successive periods of heating
in a neutral atmosphere, so as to avoid any alteration in the sur-
face composition. The growth is found to begin at a few centres,
the crystals from which it proceeds increasing in size at the ex-
pense of their neighbours. It is the larger crystals which absorb
the smaller, and the process has therefore a great similarity to the
segregation of cementite in steels, although in the present instance
the system is a homogeneous one, solvent and solute being iden-
tical. The recrystallization of a solid, the smaller grains being
absorbed by the larger, is not confined to metals, having been
observed in glacier ice at a temperature just below 0°.*

The principle of differing solubility is rejected as an ex-
planation by G. Tammann,^ who assumes that the surface
tension, which is less than the forces producing rigidity in a
crystal at the ordinary temperature, may become much more
considerable with increase of temperature. When the surface

' J. E. Stead, J. Iron Steel Inst., 1898, i. 145.

2 J. C. W. Humfrey, Phil. Tram., 1902, 200a, 225.

' G. Charpy, Bull. Soc. d'' Encouragement, 1896, [v.] 1, 180 ; Etude
des Alliages, I ; G. D. Bengough and O. F. Hudson, J. Soc. Chem. Ind.,
1908, 17, 43, 660 ; y. Inst. Metals, 1909, i. 89.

* E. HagenhsLch, Zeitsch. Kryst. Mm., 1892, 20; Rapp. Congr. Intern.
Pkys., 1900, ii. 413.

' Zeitsch. Elektrochem., 1912, 18, 584.

DIFFUSION IN THE SOLID STATE 227

tension exceeds the opposing forces, two crystals unite as two
drops of fluid would do. The hypothesis is ingeniously applied
to explain the recrystallization of strained metals.

The diffusion brought about in a heterogeneous alloy by
annealing is of greater importance from a metallographic point
of view. A cast alloy is not, in general, in a state of complete
equilibrium, the velocity of cooling under ordinary conditions
being greater than that of the reactions occurring in the solid
state. It is therefore necessary in the majority of cases to
subject the specimen to further thermal treatment in order to
bring about equilibrium, especially when it is intended to use
the alloy subsequently for the purpose of determining its
physical properties.

An alloy consisting of a single solid solution, if rapidly
cooled, shows a cored structure due to a difference of com-
position between the crystals first deposited and the layers subse-
quently deposited on them, as explained in the last chapter.
The effect of annealing in this case is to increase the molecular
mobility and thus to facilitate diffusion. The higher the tempe-
rature, the more readily the equalization of composition by
diffusion can take place, and the shorter is the period of anneal-
ing which is necessary. The cores in an a solution of copper
and tin disappear after annealing at 750° for three hours.'

If the alloy consists of two solid solutions, the annealing
process, by removing supersaturation, brings the relative propor-
tions of the two constituents to those demanded by equilibrium.
An alloy of 60 per cent. Cu and 40 per cent. Zn, rapidly cooled,
consists only of the /3 solution, which exists at the ordinary
temperature in a supersaturated or metastable state. If heated
to a temperature below the transformation point (about 740°)
crystallization of the a constituent takes place, and continues
until the proportions corresponding with the solubihty of a in (3
have been attained. By annealing for a period insufficient to
bring about complete equilibrium, proportions of u less than
the theoretical are obtained.

The annealing temperature for alloys composed of two
solid solutions should therefore be sUghtly below the transfor-
' A. Portevin, Hev. de Metallurgie, 1909, 6, 813.

228 METALLOGRAPHY

mation temperature, if the object of the annealing is to bring
the alloy into a condition of equilibrium. A lower temperature
will often suffice if the heating is continued for a longer time.

Annealing at too high a temperature may result, not only
in the production of an excessively coarse structure, but in the
separation of the crystal grains, the metal or alloy being
traversed by numerous fissures following the boundaries of the
crystalline polyhedra. Such a coarse, fissured structure is
characteristic of " burnt " steel and brass. The effect is due to
the production of gas at the boundaries, which, in forcing its
way out of the metal or alloy, separates the grains. Iron or
steel becomes " burnt " in an oxidizing atmosphere, oxygen
entering the mass and combining with the carbon present,
forming carbon monoxide, the escape of which produces the
fissures. The very similar structure observed in overheated
brass is probably due to the volatilization of a part of the zinc,
solid solutions of zinc in copper having an appreciable vapour
pressure below their melting-point, A coarse structure,
exhibiting gaps between the grains, and a pitted surface, are
characteristic of " burnt " metals.

Tempering

The process of tempering applied to steels may be regarded
as a special form of annealing, arrested at a point short of
complete equilibrium. A steel having been quenched from a
temperature above the critical point is hard, and contains
carbon in a state of solid solution. If reheated to a tempera-
ture near the critical point, the supersaturated solid solution is
resolved into its constituents, ferrite and cementite. If the re-
heating is conducted at a much lower temperature, the reso-
lution is only partial, and a part only of the carbide is set free,
the remaining carbon being still retained in solution. The
hardness and brittleness of the quenched steel are therefore
diminished, but not completely destroyed. The heating is
insufficient to cause segregation, so that a pearlitic structure is
not produced, and the carbide thrown out of solution remains
disseminated through the steel in a finely granular, or even
ultra-microscopic condition. The higher the tempering tem-
perature, the larger the quantity of carbide set free in this

DIFFUSION IN THE SOLID STATE 229

disseminated condition, and the less the quantity retained in
solution. The desired degree of spftness is obtained by control-
ling the" temperature of re-heating, which is determined in prac-
tice by the tint produced by the oxidation of the surface. The
tints observed correspond with the following temperatures ' : —

Colour. ''C.

Pale yellow 220

Straw yellow 230

Full yellow 243

Brown 255

Brown and purple 265

Purple 277

Bright blue 288

Deep blue 293

Dark blue, nearly black 316

The tints are interference colours, due to the formation of
a transparent film of oxide. In modern practice, the use of
salt baths, the temperature of which is maintained constant
at the required value, is to some extent superseding the
empirical control of the tempering process by observation of
the heated surface^ but the importance of the temper colours is
nevertheless very great. The alteration of colour is due to the
increase in thickness of the film of oxide, and this increase
must take place by diffusion within the solid. Proceeding
very rapidly at first, the process of diffusion soon reaches a
limit, so that the film does not continue to increase in thickness
to any appreciable extent after a time. Since the colour is a
very accurate indication of the temperature to which the metal
has been heated, it has been maintained- that oxidation must
cease when a certain limiting depth, dependent only on the
temperature, is reached. This is not the case ; prolonged
heating at a given temperature leads to the production of a
film corresponding in thickness, and therefore in colour, with a
higher temperature. The exact law of the rate of oxidation is
unknown. At incipient redness the film hecomes so thick as
to be brittle and detachable.

' H. M. Howe, Metallurgy of Steel, 2nd ed., i. 23 (New York, 1891).

' C. Barus and V. Strouhal, Bull, U.S. Geol. Surv., 1886, 27, 51 ;
C. Barus, Nature, 1889, 41, 369.

CHAPTER XII

THE PHYSICAL PROPERTIES OF ALLOYS

The thermal behaviour of metallic alloys with changing tem-
perature, and the microscopic appearance of the solidified
mixtures, are properties which lend themselves so conspicuously
to the systematic investigation of alloys that the term " metal-
lography " has been confined by some to these two departments
alone. There is, however, no legitimate reason for thus restrict-
ing the scope of the science. Historically, an important part
has been played by the study of the electrical conductivity and
electrolytic potential, and many other methods have been
employed from time to time. It is true that no other property
has shown so close a connection with constitution as to give
unambiguous results in all cases, but there are several, the
determination of which has high auxiliary value. In complex
instances, such as alloys containing several series of solid
solutions, it is often difficult to determine, on thermal and
microscopical grounds alone, whether inter-metallic compounds
are present or not. The occurrence of a discontinuity in some
other physical property, such as the hardness or the electrolytic
potential, will often serve to decide the point.

In addition to its value in providing auxiliary methods of
research, the study of the physical properties of alloys is of impor-
tance in quite another direction. The practical utility of alloys is
dependent on their possession of certain physical characteristics,
which in fact determine the employment in industry of alloys
in place of pure metals. It is important, therefore, to establish
such a relation between the constitution of a series of alloys —
most conveniently expressed in the form of the thermal

230

THE PHYSICAL PROPERTIES OF ALLOYS i^i

equilibrium diagram — and their physical properties, that the
latter may be predicted for any given alloy by an inspection of
the diagram. This is now in many cases possible with a fair
degree of accuracy, although the supply of the necessary data
is still far from adequate. Only a small proportion of the data
found in the literature, especially in physical journals, can be
utilized, the experiments having been performed in so many
cases with material of insufficient homogeneity. Defects, such
as porosity, exercise an enormous influence on the density,
conductivity, etc., of the specimens examined, whilst presenting
only a slight inconvenience in the microscopical study, and
being without effect in the thermal investigation. There is
little doubt that alloys intended for physical examination
should, if comparable results are to be obtained, be brought to
their maximum density by mechanical compression sufficient
to remove all pores, and should then be freed from strain by
thorough annealing. Apart from the difficulty of such an
operation with small quantities of material, the brittle and
friable qualities of many alloys render such treatment im-
possible. These considerations explain the wide divergences
between values obtained by different investigators, and pur-
porting to refer to the same alloy. It should be added that
general agreement has not yet been reached as to the means to
be adopted in measuring certain properties, notably hardness
and electrolytic potential, and results obtained by different
methods are not always comparable.

The physical properties which have been brought into
closest relation with the constitution are: (i) density; (2)
thermal expansibility; (3) hardness; (4) electrical conductivity ;
(5) thermo-electric force; (6) magnetic susceptibility; (7)
electrolytic potential. The last of these, on account of its
importance in relation to the chemical properties, will be
considered separately in the next chapter. The specific heat
and the heat of solution have also been employed, but without
any definite results. The latter of these will be discussed
briefly in connection with the electrolytic potential. The
vapour pressure of amalgams will be considered in dealing
with the molecular condition of dissolved metals.

232 METALLOGRAPHY

Density and Specific Volume

The density of a metal or alloy, that is, the mass of a unit
volume, is dependent on the temperature and also on the
mechanical condition of the body, a rolled or hammered metal,
for instance, having a different density from one cast or
annealed. The first effect of applying a pressure exceeding
a certain limit is to produce an increase of density, which
persists after the load is removed. This increase is only
apparent, and is merely due to the closing up of previously
existing pores and cavities, which are usua'ly present to an
unexpected extent, even in apparently homogeneous materials.
A pressure of 10,000 atmospheres has been found sufficient
in the majority of the cases examined to produce this effect,
the compressed substances behaving as perfectly elastic
bodies towards any further increase of pressure, however
long continued.^ This is only true if polymorphic changej^
which is brought about in many substances by the appli-"
cation of pressure, is absent. Further, it only holds good of
pressure applied equally in all directions. When a solid is
subjected to rolling, drawing, or hammering, the compression is
accompanied by change of form, and when this takes place
under great pressure, an actual diminution of density is found.
This fact is known in workshops where wire is drawn, and it
has been accurately studied in a number of cases.^ • The
meaning of the phenomenon will be discussed subsequently
in connection with the plasticity of metals. In all cases in
which a diminution of density is produced by deformation,
annealing at a temperature well below the melting-point causes
a return to the maximum density. This fact has been estab-
lished by Spring and by Kahlbaum. A metal or alloy which

' W. Spring, Bull. Acad. roy. Belg., 1883, [lii.] 6, 507.

= G. W. A. Kahlbaum, K. Roth, and P. Siedler, Zeitsch. anorg. Chem.,
1902, 29, 277 ; W. Spring, Bull. Acad. roy. Belg., 1903, 1066 ; G. W. A.
Kahlbaum, J. Chim. Phys., 1904, 2, 537; G. W. A. Kahlbaum and
E. Sturm, Zeilsch. anorg. Chem., 1905, 47, 217 ; J. Johnston and L. H.
Adams, J. Amer. Chem. Soc, 1912, 34, 563.

THE PHYSICAL PROPERTIES OF ALLOYS 233

has been thoroughly compressed by drawing or hammering, and
subsequently annealed, is therefore likely to give the true density.
It must be noted that even this precaution does not exclude
every source of error, as the original metal may contain
bubbles of gas, which will be compressed to very small
dimensions, but will recover their original volume on annealing,
causing the observed density to be too low.

Although the density of a body is properly the mass of
unit volume, and the specific gravity is the ratio of the mass of
the body to that of an equal volume of water, the two magni-
tudes -are expressed by the same number when metric units are
employed, and the Archimedean method, which consists in
comparing the weight of the specimen with that of the water
which it displaces, is therefore the best adapted to the study of
the density of alloys. The principal error in this method is due
to the suspending hair or wire. It has been found * that the
most uniform results are obtained when the specimens are
suspended by a platinum wire, o'32 mm, in diameter, carefully
platinized by electrolysis, washed, and dried, all contact with
its surface being avoided. The specific gravity is then given by
the formula

w
s =. (G -X) +A.

in which s is the specific gravity referred to water at 4°, w and
Wi are the weights of the specimen in air and in water respec-
tively, G is the density of the water at the temperature of the
experiment, and X the density of the air.

If the alloy is one which is acted on by water, the weigh-
ing is performed in some inert liquid, such as petroleum,
the density of the liquid being inserted in place of G in
the above formula. Liquid amalgams are weighed in a
pyknometer.

When comparing the members of a series of alloys, how-
ever, the magnitude selected is not the density, but its reciprocal,
the specific volume. If a series of alloys consists, as in the
simplest case, entirely of conglomerates of crystals of the two
' Kahlbaum and Sturm, loc, cit.

234

METALLOGRAPHY

components, the specific volume of any alloy will be that
calculated from the specific volumes of the component metals
and the proportions in which they are present, whilst there is no
such direct relation between the densities. The specific volume-
concentration curve is therefore, in the case of conglomerates,
a straight line. This is found to be true with a fair amount
of accuracy when percentages by weight, and not atomic
percentages, are taken as abscissae.^

In the table, which has been constructed from Maey's
figures, the constants of the linear equations, v=a-\-bp, are
given for a number of pairs of metals, the alloys of which are
known, from other evidence, to consist of conglomerates of
the pure component metals, or of solid solutions of limited con-
centration. In the equation, v is the specific volume of an alloy
and p the percentage by weight of the second metal, a being the
sp. vol. of the first metal, and b = the difference of the sp. vols,
of the two metals^ I GO. The fourth column contains the
values of rooAw/z;, where Az* is the greatest deviation from
the calculated value shown by any alloy of the series, and v is
the specific volume of that alloy. iooAz'/» therefore gives the
greatest percentage deviation observed.

looAt'

Metals.

a

*

V

Bi-Cd

OTO181

0-0001373

— o-i

Bi- Sn

o-ioi8t

0-0003530

+ 1-0

Cd-Sn

0-II554

0-0002156

-(-0-5

Pb - Cd

0-08791

0-0002763

-1-0-4

Pb -Sb

o-o879i

00006106

-1-0-9

Pb -Sn

0-08791

0-0004919

-1-0-9

Pb - Ag

0-08791

0-0000760

+ o■^

Ag- Bi

0-09550

0-0000630

-0-4

Sn - Zn

0-13710

o-ocKX)400

-1-0-5

Hg-Pb

0-07368

0-0001422

- i-o

Hg-Sn

0-07368

0-0006345

-09

' A number of determinations were made by A. Matthiessen, Phil.
Trans., i860, 150, 177 ; who, however, expressed his results as densities.
The largest mass of data is found in a series of papers by E. Maey, Zeitsch.
physikal. Ckem., 1899, 29, 119; 1901, 38, 289, 292; 1904, 60, 200.
Maey recalculated the older figures of Matthiessen and others, expressing
the results in terms of specific volume, and also made many new determi-
nations.

THE PHYSICAL PROPERTIES OF ALLOYS 235

Considering the difficulties of the determinations, the
agreement within i per cent, must be considered satisfactory.

A solid solution is formed, as a rule, from its components
without change of volume.^ The following instances of metals
forming unbroken series of solid solutions indicate that in
these typical alloys at least the volume changes are in-
appreciable : —

a

*

looAz/

V

Au - Ag
Ir-Pt

o'osigi
0-0446 I

o'ooo4309
o'ooooigo

+ 0'2
±0'2

The method of specific volumes is therefore incapable of
distinguishing between conglomerates and solid solutions. On
the other hand, series of alloys containing inter-metaUic com-
pounds show specific volumes which at certain concentrations
deviate in a marked degree from the values calculated by the
linear relation. A stable compound, formed with development
of heat, might be expected to have a smaller volume than that
of its components. It is, however, remarkable that in several
cases a marked expansion has been observed, instead of a
contraction, at compositions corresponding with compounds,
although it may prove that some of the alloys of which this is
reported were porous, and thus gave too low a density. In the
table on p. 236, some cases are collected of pairs of metals, the
alloys of which show marked contraction or expansion. The
second column gives the composition of the alloy which exhibits
the greatest deviation, the third column gives the molecular
formula most nearly corresponding with that composition, and
the last column, as before, the greatest percentage deviation of
the specific volume from that demanded by a linear relation.

In many series, as in the alloys of zinc and copper, the
deviations, although considerable, do not indicate any distinct

' Solid solutions of isomorphous salts show a strictly linear relation
between specific volume and concentration. J. W. Retgers, Zeitsch.
physikal. Chem., 1889, 8, 497.

236

mmtallography

Formula.

V

Ag- Sn
Cu- Sn
Sb -Fe
= Al - Sb
Zn-Sb
Ag-Zn

70'8 per cent. Ag
39-0 „ Sn
40 „ Fe
18 „ Sb
45 .. Zn
71 ,, Zn

AgjSn

CujSn

Fe,Sb/

AlSb

Zn,Sbj

AgZn,

-5

-9

- 14

+ 24

+ s

- 2

break in the curve at a definite composition, and it is impos-
sible to assign a formula on the ground of density measure-
ments. Assuming it to be true that a discontinuity in the
specific- volume curve always indicates the existence of a com-
pound, it does not by any means follow, as supposed by Maey,
that the discontinuity will occur exactly at the composition of
the compound. This is only true if solid solutions are not
formed. This is, however, rather the exception than the rule,
the majority of inter-metallic compounds being able to retain
a greater or less quantity of one or both components in solid
solution. The effect on the specific volume may be illustrated
by an example. In Fig. 79, the upper part represents the
thermal diagram of a series of alloys of the metals A and B
containing a compound, C, which is capable of forming solid
solutions up to the limiting concentrations D and E respec-
tively. We will assume that B has a greater specific volume
than A, and that the compound C is formed with contraction.
All solid alloys between A and D will consist of conglomerates
of A and the saturated solution D, their specific volumes will
therefore follow a linear law, as indicated in the lower half of
the diagram. At D the last crystals of A disappear, and between
D and E the alloys consist only of the solid solution, of pro-
gressively changing concentration. As the percentage of B,

' Maey gives the formula FeSb for this compound, placing the dis-
continuity at 33'8 per cent. Fe. The figures agree better, however, with
the formula FejSbj, the curve at this part of the diagram being irregular.
The existence of FejSbj is probable from the thermal results (N. S. Kurna-
koffand N. S. Konstantinoif, Zeitsck. anorg. Chem., 1908, 88, l).

^ E. van Aubel, Compt. rend., 1 90 1, 132, 1266.

THE PHYSICAL PROPERTIES OF ALLOYS 237

the metal of greater specific volume, increases, the specific
volume of the solid solution increases also. From E to B the
alloys consist of conglomerates of B and the saturated solid
solution E, and the specific volume consequently varies again

\/ D

c

E \/

A

/

E + B

A+ D

Solid,
solutions
D-E

A

1 !
1 .
1 1

1
1
1

1 1

B

=^B

FiO. 79.— Constitution and specific volume.

in a linear manner. The specific-volume curve, therefore,
exhibits two discontinuities, corresponding, not with the com-
pound C, but with the limits of saturation of the solid solution.
From an inspection of the curve, nothing could be inferred as
to the composition of the compound, except that it lies bet\^een

238 METALLOGRAPHY

the limits D and E. The rule that a discontinuity in a physical
property indicates the appearance of a new solid phase, and
does not without further evidence yield the formula of the
compound present, applies to the electrical conductivity, hard-
ness, etc., and especially to the electrolytic potential of alloys,
as well as to their density. Neglect of this truth is responsible
for the appearance, in chemical literature, of a very large
number of inter-metaUic compounds which have no real
existence.'

To sum up, the specific volume varies with the composition
within a series of alloys as follows : —

1. In conglomerates of two constituents, the variation is
linear. The small deviations observed are probably due to
want of homogeneity.

2. In a series of solid solutions, the variation is almost
linear. Further investigation may perhaps reveal some
exceptions.

3. In a more complex series, discontinuities occur at each
point of appearance of a new solid phase. Phases containing
inter-metallic compounds have usually a smaller, but in a few
cases a larger, specific volume than their components.

Thermal Expansibility

The change in the coefficient of expansion with change of
composition in a series of alloys has remained practically
unused as a method of investigating constitution. There is no
reason why the method should not be applicable, and it is
probably only the troublesome nature of the experiments,
which require a well-equipped laboratory for their performance,
that has hindered its use.^ The expansion with increase of
temperature and contraction on cooling has, however, rendered

' C. H. Desch, J. Inst. Metals, 1909, i. 227,

^ For a few such measurements, see H. Le Chatelier, Etude des AUiages,
387 (Paris, 1901). Alloys of copper with antimony and with aluminium
were studied by the optical methpd, maxima being found at points in the
expansion curves corresponding with the compounds CujSb and CujAl.
A few measurements of this kind were made by A. Matthiessen, Phil.
Traits., 1866, 156, 861.

THE PHYSICAL PROPERTIES OF ALLOYS 239

great service in the detection and fixing of critical or trans-
formation points in metals or alloys capable of existing in
polymorphic forms. A substance, such as platinum, which may
be heated through a great range of temperature without under-
going change, increases in volume in a regular manner, the
volume at a given temperature being given by a relation of the
form

V = Vo(i + a^ + ^f)

the curve being a smooth parabola. Any polymorphic change
taking place in the metal is indicated by a discontinuity in this
curve at the temperature of the change. Observations of
expansion are particularly valuable as a check on thermal
analysis in cases of polymorphism, since a change accompanied
by a very small development of heat may be accompanied by
a marked alteration in volume, and vice versd. When volume-
and energy-changes are both absent, there is a strong presump-
tion against polymorphism.

At moderate temperatures, volume-changes are most con-
veniently measured in a dilatometer. The metal is enclosed
in a glass (or, better, quartz) bulb fitted with a stem. The
bulb and part of the stem are filled with a liquid which does
not act on the metal, and the level at each temperature is read
by means of a scale and telescope.' This method has been
used to determine the transition-point between grey and white
tin, which is thus fixed at 18°, in agreement with the electro-
lytic results.^ It has also proved useful in the investigation of
amalgams.'

When small bars of the metal or alloy can be accurately
prepared and shaped, the expansion is best measured on a
small prism, one end of which is polished and placed in contact
with an optically worked, very flat convex lens of glass. The
Newton's rings produced when a beam of monochromatic light
is reflected from the metal after passing through the lens alter

' For a description of the method, see Ostwald and Luther, Physico-
Cheniical Measurements, and Findlay, Phase Rule, 293.
^ E. Cohen, Zeiisch. physikal, Chem., 1899, 30, 601.
' H. Bijl, Zeitsch. physikal. Chem., 1902, 42, 64I.

240 METALLOGRAPHY

in diameter as the temperature is raised, and their diameter is
measured by suitable optical means."

The dilatation method has been most extensively applied
in connection with the transformation-points of the alloys of
iron. H. Le Chatelier {loc. cit) used a method depending on
the difference in expansion of the metal under investigation
and a standard material (hard Sbvres porcelain). The samples
examined were 70 x lo X 5 mm. in size, and their displace-
ment relatively to the porcelain standard was measured by the
movement of a mirror.

A similar method, very simple in its application, and suit-
able for work in a metallographic laboratory, was employed in
detecting the abrupt changes in length which take place in the

C

-Simple form of dilatometer.

cooling of iron containing carbon.^ A bar of the metal under
investigation, A (Fig. 80), is placed between two porcelain
rods, BC and EF, the first of which is fixed rigidly at D, whilst
the second is free lo move in a horizontal plane about a pivot
at G. The ends of the rods are pressed against the specimen
by a spring at H. Any movement of F relatively to C, due to
an alteration in length of A, is communicated to the mirror K,
attached to a vertical torsion fibre. A ray of light reflected by
K on to a scale magnifies the movement of F. The bar A is
heated in a furnace, and simultaneous readings of the tempera-
ture and the scale deflection are made. The apparatus is easily
rendered self-recording.

An apparatus susceptible of greater accuracy, and yielding
an autographic record lending itself well to the recognition of

' H. Le Chatelier, loc, cit.

' G. E. Svedeliiis, PMl. Mag., 1898, [v.] 46, 173.

THE PHYSICAL PROPERTIES OF ALLOYS 241

Porcelain

critical points, has been devised * in which the expansion of the
specimen is compared with that of a tube of pure silver of the
same length. The arrangement of the bodies to be compared
is shown in Fig. 81. The outer tube is of hard porcelain.
Within it rests a tube of silver
fitted with a collar, on which rests
a porcelain tube of the same
diameter. The specimen to^ be
examined stands vertically inside
the silver tube, and has the same
height as it. A porcelain rod rests
on the specimen. This rod and
the inner porcelain tube are con-
nected each with the short arm of
a lever carrying a recording-point,
which presses on a revolving drum
on which paper is fixed. Each
point therefore traces a separate
curve on the paper. One of these
curves records the expansion of
the silver. comparison tube, the other that of the body studied.
Any irregularity of expansion of the porcelain is eliminated
from the results, as both curves are equally affected. The
apparatus might easily be adapted to trace a single curve
corresponding with the differences in the movements of* the
two bodies. The outer tube stands in a vertical furnace,
and the temperature is measured by a pyrometer introduced
into the expansion apparatus.

Fig. 82 represents some of the results obtained with iron
containing different quantities of carbon.^ The change on
heating is marked by a contraction, which is near 900° for
almost pure iron, becoming lower with increasing carbon
content, whilst alloys containing an appreciable amount of
carbon exhibit a strong contraction near 700°, this being the
only change in the 0*64 per cent. C steel.

' R. von Sahmen and G. Tammann, Ann. Physik, 1903, [iv.] 10, 879.
" G. Charpy and L. Grenet, Bull. Soc. (t Encouragement, 1903, 102,
464. 883.

T.P.C. R

Fig. 81.— Sahmen and Tam-
mann's dilatometer.

242

METALLOGRAPHY

The volume-changes in an alloy which undergoes mole-
cular changes on heating are sometimes very large. Thus
Rose's fusible alloy expands up to 44°, then begins to contract,
the contraction continuing until 69° is reached, and then
expands, the prolongation of the original curve not being

200 400 600 aoo /ooo °C
Fig. 82, — Expansion curves of steels.

reached until 98° (the melting-point). The volume at 69° is
much less than that at 0°.^

The change of volume which takes place on the passage
from the solid to the liquid state has been measured in a few
cases. As a rule, a metal occupies a greater volume at its
melting-point in the liquid than in the solid state. Bismuth
and certain alloys contract on melting, and their melting-point,
like that of ice, is therefore lowered by pressure.

The percentage increases of volume on melting are : "

Lead . .

• 3"39

Potassium .

. 2-6

Cadmium .

• 472

Mercury

. 3-67

Tin . . .

. 2-8o

Bismuth

•-3"29

Sodium . .

■ 2-5

' O. D. Chwolson, Lehrb. der Physik., vol. iii.

' G. Vincentini and D. Omodei, Atii R. Accad, Set. Torino, 1887,
23, I ; M. Topler, Ann. Phys., 1894, [iii.] 63, 343.

THE PHYSICAL PROPERTIES OF ALLOYS 243

Hardness

Although the hardness of a soUd is one of its most im-
portant physical characters from a practical point of view, it is
one to which it is difficult to assign a definite scientific mean-
ing, and several quite distinct magnitudes are in fact com-
prehended under the same general term. So far, it has not
been found possible to express hardness in any scientific unit,
and the numbers used to describe the hardness of a substance
are therefore purely empirical, and refer only to one particular
method of testing. These methods may be classified accord-
ing to the kind of resistance they measure —

1. Resistance to scratching or cutting. This is hardness
in the mineralogical sense,

2. Resistance to indentation, when stressed beyond the
elastic limit by a concentrated load applied (a) gradually, {b)
suddenly.

3. Elastic reaction.

I. Sclerometric methods. The mineralogist uses an arbi-
trary scale of standard minerals, so chosen that each member
of the series is scratched by all those minerals above it, and
scratches all those below it. Mohs' scale ranges in this way
from I (talc) to 10 (diamond). A more exact measure of the
mineralogical hardness is obtained by scratching with a
hardened point (diamond or glass-hard steel) under a fixed
load, and either measuring the width of the scratch or deter-
mining the loss of weight by abrasion after a given number of
passages of the point. The latter method is not employed in
metallography. The sclerometer takes many forms, of which
that devised by T. Turner '■ is one of the most generally used.
A diamond point attached to a lever arm is loaded with a
variable load, and is drawn across the polished surface of the
specimen, the " hardness number " being the number of grams
required to press the point so heavily as to produce a " normal
scratch," defined as a scratch which is just visible as a dark
line on a bright surface. There is naturally considerable room
for error in judging the proper depth of scratch, especially on
' Proc. Birmingham Phil, Soc, 1886, 5, 291.

244 METALLOGRAPHY

very different materials, and the method is mainly useful in
comparing samples of similar materials, such as steels. It
gives very concordant results in practised hands.

Martens' form of sclerometer ' is more generally useful, and
has been employed in most scientific investigations of the
hardness of alloys. In this case the point, usually loaded with
a load of 20 grams, is stationary, whilst the specimen, with its
polished surface horizontal, is drawn to and fro by a traversing
table. The breadth of the scratch is measured, either by
means of a microscope with micrometer eyepiece, or by
photographing the scratch under a known magnification, and
measuring the photographic image.^ The hardness number is

given by the relation H = - , where a is the breadth of the

scratch in mm. When the alloy consists of a conglomerate of
two solid phases, it is still possible to obtain useful results as
to the hardness, provided that the grain of the mixture is fine.
The breadth of the scratch observed is then actually an average
value, the microscope showing that the scratch is made up of
wide and narrow portions, but a consistent reading is obtained
without difficulty if the grain of the alloy is not too coarse. An
improved form of Martens' sclerometer, mounted on the stage
of a microscope, is described by V. Poschl.*

In both these forms of sclerometer the motion of the
diamond point relatively to the specimen is one of translation,
but a movement of rotation may equally well be employed.
This is the principle on which Jaggar's microsclerometer, a
very delicate instrument designed for mineralogical purposes,
is based.* This instrument is really a minute drill, mounted
as an attachment to a microscope. The point, a cleavage
tetrahedron of diamond, is rotated by hand or by clockwork
at constant speed under a constant load. The depth is
measured by a small slip of glass, ruled with lines o'oi mm.

• A. Martens, Mitt. k. techn. Versuchs-Anst., 1890, 8, 236.

' N. S. Kurnakoff and S. F. Schemtschuschny, Zeitsch. anorg. Chem.,
1908, 60, I.

• Die Hdrte derfesten Korper (Dresden, 1909).

• T. A. Jaggar, Amer. J. Set., 1897, [i^-] 4, 399.

THE PHYSICAL PROPERTIES OF ALLOYS 245

apart, and cemented to the lever arm bearing the point. The
glass is slightly inclined to the vertical, with its rulings
horizontal. The microscope is focussed on one of the lines,
and drilling is continued until the next ruling just comes into
sharp focus. The drill has then penetrated to a depth of
o'oi mm. The hardness number is the number of revolutions,
as recorded on a dial counter, required to drill to that depth
under a load of lo g. This instrument is not only very
accurate in its indications, but has the advantage of operating
on an exceedingly small area, so that the individual con-
stituents of an alloy may be separately examined if visible
in the same micro-section.'

2. Indentation method. Brinell's test,' which is now the
most important hardness test in technical practice, consists in
pressing a hardened steel sphere, which will not undergo
deformation, on to the surface of the metal to be tested, and
measuring the depth of the circular indentation. The hardness
number is the pressure per unit area at the centre of that area
required to produce the indentation, and within certain limits
this is found to be independent of the size of the ball and of
the pressure applied. The theory of this form of test has been
discussed by Auerbach.'

In the practical application of the test, a hardened steel
ball (manufactured for use in bearings), 10 mm. in diameter, is
pressed on the surface of the metal under a known load. The
simplest form of apparatus is a lever, pivoted at one end and
loaded at the other with a known weight, the lever resting on
the ball near to the fulcrum. A more convenient appliance is
a small hydraulic press, the cylinder of which is filled with oil.
Pressure is applied by a pump, and is measured by a separate
cylinder, the piston of which is loaded by a lever carrying a

' Thus H. C. Boynton, J. Iron Steel Inst., 1906, ii. 287 ; igo8, ii.
133, examined the constituents of steel with this instrument, obtaining
numbers ranging from 460 for pure electrolytic iron to 125,480 for cementite,
Fe,C.

' Rajip. Congr. intern. Mithodes cPEssai, Paris, igcxj.

» F. Auerbach, Ann. Phys., 1891, [iii.] 43, 60 ; 1892, [iii.] 45, 262,
277 ; 1900, [iv.] 3, 108.

246

METALLOGRAPHY

sliding weight.' The load being increased to a fixed amount,
depending on the nature of the metal or alloy to be examined,
the diameter of the circular indentation produced is measured
by means of a micrometer. A few results obtained in this
way are given in the following table : —

Material.

Maximum pres-
sure on 10 mm.
ball.

kilog.

500

)j

j>
»i

3)
J)

500

>>

Diameter of
impression.

Hardness number.

Copper

Silver

Antimony ....

Gold

Zinc

Aluminium.

Tin

Lead

Phosphor-bronze .
Bell metal ....

Brass

Phosphor-tin . . .
Antifriction 1

metals > .

(Pb, Sn, Sb))

mm.
2-90
3-25
3-35
3-60

3-6S
4'oo
6-25
6-30

2-20
2-2S
3-15
5-45

4-S

74-0

59-0

48-0
46-0
380
14-5

57

130-0

124-0

630

«97

37-23

The dependence of Brinell's hardness numbers on the
mechanical properties of the metal have been studied, and
numerous relationships, of great technical importance, have
been discovered.^

3. Elastic reaction method. A somewhat remarkable
method, based on the rebound of a hard body from the surface
to be tested, is that devised by Shore.' The instrument, known
as the scleroscope, is illustrated in Fig. 83. The specimen

a, having been squared up and polished, is placed on the anvil

b, which is provided with levelling screws. The tube c has a
ground lower edge which is applied closely to the specimen,

' E. Preuss, Baumateriahnkundt, 1907, 295 ; A. Wahlberg, J. Iron
Steel Inst., 1901, i. 243.

2 A. Wahlberg, J. Iron Steel Inst., 1901, i. 243 ; ii. 234 ; H. Le
Chatelier, Rev. de Metallurgie, 1906, 3, 689.

' A. F. Shore, American Machinist, 1907, 30, ii. 747 ; see also
E. Meyer, Physikal. Zeitsch., 1908, 9, 66 ; R. de FreminviUe, Rev. de
Mitallurgie, 1908, 5, 329 ; J. F. Springer, Iron Age, 1908, 82, 555.

THE PHYSICAL PROPERTIES OF ALLOYS 247

the plumb-line d being used to secure an exactly vertical
adjustment of the tube. A small steel ball, or cylinder with
rounded lower end, is raised to the top of the tube by air
suction with the rubber ball e and then released, falling from
a height of exactly 25 cm. on to the specimen. The height of
the rebound is measured on the graduated scale of the tube.
This can be done very accurately by means of a reading lens,
supported on a sliding stand in front
of the apparatus. The approximate
height of rebound is found by pre-
liminary trial, and the lens is then
brought to the required position,
after which the exact rebound can
be determined with ease. Successive
readings give perfectly concordant
results. The apparatus is very well
made and convenient in use.

A thick mass of hard metal will
give under such circumstances a
rebound of 95 per cent, of the fall;
with a small specimen the rebound
is rather of the order of 65 per cent.
In studying a series of alloys by this
method, all the specimens should be of equal size, and should
be well bedded on the anvil. Under these circumstances
the results may be considered as strictly comparable. Placing
the specimen on a felt or rubber support gives an increased
rebound.

Different as these methods of determining hardness are
from one another, the relative order of hardness revealed by
them when a number of metals and alloys are compared is
strikingly similar, if a few exceptional cases be excluded. The
nature of these exceptions may be gathered from the behaviour
of certain non-metallic substances. A soft but resilient sub-
stance hke indiarubber, offering an insignificant resistance to
the Brinell test and readily cut by a point, gives a higher
result with the scleroscope than a mild steel. Different kinds
of wood also give results which are inconsistent with their

Fig. 83. — Shore's
scleroscope.

248

METALLOGRAPHY

behaviour towards cutting tools. Amongst metals and alloys,
the scleroscope indicates greater hardness in cold-worked
specimens than the other methods, that is, the elastic hardness
of a cold-worked metal is not the same as its mineralogical
hardness, or resistance to abrasion. Annealed and hard-drawn
copper differ little in resistance to a diamond point, but behave
very differently under the Brinell or scleroscope tests. As
regards steels, it would seem that Brinell's method gives
results most nearly according with practical experience as to
the- wearing power of the steel.'

In the following table, tests on a number of materials have
been compared,^ and the agreement between the results obtained
by different methods is seen to be remarkably good. Cold-
worked metals have been excluded. The hardness numbers
have been reduced to a uniform scale to facilitate com-
parison : —

Metal.

Turner's

Shore's

Brinell's

sclerometer.

scleroscope.

ball test.

Lead

lO

I

I'O

Tin . . .

2-5

3

2'S

Zinc. . .

6-0

7

7'S

Copper .

80

8

Soft iron

15-0

12

14'S

Mild steel .

21-0

22

16-24

Soft cast iron

21-24

24

24-0

Rail steel .

24-0

27

26-35

Hard cast iron

36-0

40

35 'o

Hard white iron

72'0

70

75'o

Hardened steel

—

95

93 "0

It has long been known that the hardness of a metal bears
an intimate relation to its atomic volume, the softest metals
being those of greatest atomic volume.' The rule is an
approximate one only, as data as to the hardness of pure
elements, determined under strictly comparable conditions, are
not forthcoming in sufificient quantity to establish an exact
quantitative relation. Consideration of the "internal pressure "

' E. Maurer, Metallurgie, 1909, 6, 33.

' T. Turner, J. Iron Steel Inst., 1909, i. 426,

' S. Bottone, Chem. News, 1873,27, 215.

THE PHYSICAL PROPERTIES OF ALLOYS 24^
of metals has led to the conclusion that the hardness should be
proportional to the quantity -g where v is the atomic volume,

V

and a is van der Waal's constant.'

The hardness of a metal may be very greatly altered by
alloying with another metal. This is, for instance, the object
of alloying gold and silver for coinage purposes, the hardness,
and therefore" the resistance to abrasion in wear, being thereby
increased. The way in which the hardness is changed will
depend on the nature of the alloy formed, thus —

1. The two metals are mutually insoluble in the solid state,
all their alloys being conglomerates of the pure components.
The hardness is then nearly a linear function of the composition.
It is not exactly so, since a fine-grained eutectic, composed
of a soft and a brittle metal, has a hardness in excess of that
calculated from its composition, on account of the mutual
support offered by the constituents. The values obtained by
sclerometric methods, however, approach very closely to those
calculated according to the rule of mixtures.

2. The metals form a continuous series of solid solutions.
The variation of hardness is expressed by a smooth curve
having a pronounced maximum at a composition not far from
that corresponding with equal weights of the component metals.
A typical example is furnished by the alloys of silver and gold,
shown in Fig. 84. The ordinates are sclerometric hardness
numbers. An alloy composed of equal weights of gold and
silver has a hardness which is practically twice as great as that
of either of the pure metals.'' Alloys of gold and copper, and
of copper and nickel, present similar maxima.

If the two metals form two series of solid solutions separated
by a gap, the hardness curve must present a change of slope at
each fimit of saturation, being rectilinear between them. This
is true of the alloys of silver and copper.'

» I. Traube, ZeiiscA. anorg. Chem., 1903, 34, 413 ; 1904, 40, 377 ; Ber.,
1909, 42, 1594; C. Benedicks, Zeitsch. physikal. Chem., 1901, 36, 529.

' Kuinakoff and Schemtschuschny, loc. cit.

' N. S. Kurnakoff, N. A. Pushin, and N. Senkowsky, Zeitsch. anorg.
Chem., 1910, 68, 123.

250

METALLOGRAPHY

3. An inter-metallic compound is formed. As a general
rule, such compounds are harder than their constituents. A
familiar example is the compound CugSn, which is exceedingly
hard and brittle, although its components are soft.

O 20 1-0 60 BO 100 YoAu,

Fig. 84. — Hardness of silver-gold alloys.

A series of alloys containing only a single compound, such
as the alloys of magnesium with lead, tin, or silicon, from
which solid solutions are absent, has a simple hardness
diagram, consisting of two straight lines intersecting at the
composition of the compound. The series, in fact, comprises
two distinct series of conglomerates, within each of which the
variation of hardness is rectilinear. But should solid solutions
occur amongst either series, the curve in that region deviates
from a straight line. The copper-tin alloys were found by
Martens {/oc. cit.) to present a maximum hardness correspond-
ing with the compound CujSn, the curve having a sharp peak
at this point. A compound which forms solid solutions with
both components has its hardness increased by the addition of
either metal, and is thus marked by a cusp on the curve
directed downwards. The alloys of magnesium and cadmium
illustrate this condition.^

A sharp peak or cusp in the hardness-composition curve of
a series of alloys is a sure indication of the existence of a
compound at that point. A rounded maximum has not the

' G. Urazoff, Zeilsch. anoyg. Chem., 1911, 73, 31;

THE PHYSICAL PROPERTIES OF ALLOYS 251

same significance, as it may be due to the formation of a solid
solution of increased hardness. A sudden change in the
direction of the curve indicates the appearance of a new
phase, the nature of which must be determined by other
methods.^

Electrical Conductivity

The specific electrical resistance of an alloy, or its reciprocal,
the specific conductivity, has long been recognized as being
intimately connected with the internal constitution. The
extensive researches of Matthiessen '' are, in fact, anterior to
the introduction of the thermal method, and have played an
important part in the theoretical development of the subject.

According to Drude's theory of metallic conduction,' the
passage of a current through a metal takes place through a
transport of electrons. The conductivity depends, then, on
two factors, the number of electrons present in unit volume
and the frictional resistance to their movement. The first of
these is not susceptible of measurement, but has been calculated
indirectly from the optical properties,* and also from the
thermo-electric properties. The volume-concentration of the
electrons is little affected by temperature, as is shown by a
comparison of the optical constants of metals at the temperature
of liquid air, and at 800°.' Since the electrical conductivity
changes very rapidly with the temperature, this variation must
be due, on Drude's theory, to variation of the second factor,
the frictional resistance. This property, again, cannot be

' A good example of the hardness curve of a complex system is given
by the alloys of magnesium and silver ; W. J. Smirnoff and N. S.
Kurnakoff, Zeitsch. anorg. Chetn., 1911, 72, 31.

« A. Matthiessen, Phil. Trans., 1858, 148, 383; i860, 160, 161 ; Phil.
Mag., 1861, [iv.]21, 107 ; [iv.]22, 195 ; A. Matthiessen and M. Holzmann,
Phil, Trans., i860, 150, 85 ; A. Matthiessen and C. Vogt, ibid., 1864,
154, 167 ; Phil. Mag., 1862, [iv.] 23, 171.

' P. Drude, Ann. Physik., 1900, [iv.] 1, 566; 3, 369; 1902, [iv.].
7, 687.

< P. Drude, Ann. Physik., 1904, [iv.] 14, 936.

' R. Schenck, Physikalische Chemie der Metalle, Halle, 1909.

252 METALLOGRAPHY

measured directly. It bears an obvious relation to the
hardness, and it is not surprising to find, therefore, that
hardness and electrical conductivity exhibit a close parallelism
■ in series of alloys.

Electrolytic conduction does not occur in metallic alloys,
and there is no continuous transition from the one mode of
conduction to the other.

The methods of measuring the conductivity of a metal or
alloy belong rather to physics than to metallography. The
principal difficulty which confronts the metallographist who
wishes to examine the conductivity of a series of alloys is
that of obtaining suitably homogeneous specimens* for measure-
ment. Alloys which it is possible to draw into wire are
examined with ease, and there is also little difficulty in forging
small rods of malleable alloys, but a large proportion of speci-
mens are necessarily examined in the cast state, and in a great
number of cases the brittleness is such that it is not possible
to reduce the cast specimens to accurate form by cutting or
filing. Hard metals, if not too brittle, may be ground to
shape on a wet emery wheel with light pressure. Very brittle
alloys will not bear this treatment, and the experimenter must
be content with cast rods. When the melting-point is not too
high, smooth rods of uniform diameter may be obtained by
the device of drawing the molten metal with the aid of a
pump into previously heated glass tubes, coated internally with
lamp-black. The glass is afterwards carefully broken away
from the rod.^ Cast specimens should always be examined
microscopically for homogeneity, since gas-cavities lead to
errors of the sarfle nature as in determinations of density.

The cause of the increased resistance of alloys as compared
with that of their component metals has been much discussed.
It has been suggested by Lord Rayleigh'' and also by Liebe-
now,^ that the Peltier effects at the numerous boundaries
between the structural components give rise to thermo-electric
currents, which have the same effect as an increased resistance.
This hypothesis appears to explain the facts fairly well so far as

' N. J. Stepanofif, Zeitsch. anorg. Chem., 1908, 60, 209.
' Nature, 1896, 64, 154; Collected Papers, iv. 232.
' C. Liebenow, Zeilsch. Elektrochem, 1897, 4, 201,

THE PHYSICAL PROPERTIES OF ALLOYS 253

concerns alloys which are mechanically heterogeneous, but its
application to solid solutions presents difficulties. Theoretically,
the Peltier effects may be supposed to occur even when the
state of subdivision of the components is a molecular one, as
in a solid solution. But the remarkable form of the typical
U-shaped curve is hardly to be explained in this way, and the
hypothesis must be admitted to fail in this important class of
cases.'

An attempt has been made to test Rayleigh's suggestion
experimentally.^ Assuming the resistance to be due to Peltier
effects, the resistance offered to the passage of an alternating
current should be distinctly less than that offered to a direct
current, as the time allowed during each alternation would
probably be insufficient to allow the temperature to become
equalized, and the back E.M.F. would not be set up. In a
series of experiments with German silver, platinum-iridium,
platinum-silver, and similar alloys, it was found that no false
resistance could be detected in this way, even when a rapidly
alternating current from an induction coil was used. All the
alloys examined consisted of solid solutions, and it does not
appear that alloys composed of conglomerates have been tested
by this method. The evidence is not conclusive, but so far
experimental evidence has not been obtained in favour of
Rayleigh's hypothesis, and it may perhaps ' prove that the
application of Drude's views on metallic conduction may bring
more light into the question.

When it is desired to express the dependence of the
electrical properties on the composition of alloys, either the
specific resistance or its reciprocal, the specific conductivity,
may be selected as the magnitude for comparison. The
conductivity is usually to be preferred, as giving the simpler
relationships.

The specific conductivity of conglomerates is very nearly
proportional to their volume composition. The conductivity-
concentration curve of alloys forming a simple eutectiferous

» R. Schenck, Metallurgie, 1907, 4, 161 ; Pkysikal. Zeitsch., 1907, 8,
239 ; R. Guertler, Zeitsch. anorg. Chem., 1907, 54, 58.
' R. S. Willows, Phil. Mag., 1907, [vi.] 12, 604.

254 METALLOGRAPHY

series, that is to say, closely approximates to a straight line,
if the concentration be expressed, as was done by Matthiessen,
in percentages by volume. The reason for this will be seen if
we consider that the constituents of a mechanical mixture
conduct independently, so that we may imagine them
separated, drawn into wires of equal length, and placed side
by side in a bundle. The conductivity of the whole bundle
is then the sum of the conductivities of its constituent wires,
and as the conductivity of each wire is proportional to its
cross-section, and the wires are of equal length, it is the
volume of each constituent which must be taken into account
in the linear relation. Peltier effects, the production of back
E.M.F. owing to heating at the contact of crystals of different
kinds, are here neglected, but they may become appreciable,
giving rise to deviations from the linear relation, in the sense
of giving a conductivity lower than that calculated from the
conductivity of the component metals and the volume-composi-
tion. The small deviations observed in alloys of lead and
cadmium, and of zinc and cadmium, are of the order required
by this thermo-chemical explanation.

A very different condition presents itself in those alloys
which consist of soUd solutions. The conductivity of a pure
metal is lowered to a remarkable degree by the addition of
small quantities of a second metal which it is capable of
retaining in solid solution. In the graphical representation the
conductivity curve falls very steeply for small percentages of
the second metal, the slope then becoming less steep. In
similar manner the other end of the curve shows a steep fall,
the intermediate portion being gently curved. The entire con-
ductivity curve has therefore a characteristic U shape, being
continuous throughout. The curve for the alloys of gold and
silver, determined by Matthiessen, is typical of this condition
(Fig. 85). Alloys containing nearly equal volumes of the two
metals have a conductivity which is only about one-fifth of that
of gold, the less conducting of the two components,^ The

' The determinations of V. Strouhal and C. Barus, Abh. k. bohm. Ges.
Wiss., 1884, [v.] 12, No. 14, agree with those of Matthiessen. A large
number of curves, recalculated to a uniform scale, are collected in the

THE PHYSICAL PROPERTIES OF ALLOYS 255

case of the alloys of copper and nickel is perhaps even more
striking, on account of the very unequal conducting powers of
the two metals. Although the conductivity of copper is about
seven times that of nickel, the addition of copper to nickel
produces a rapid diminution of the conductivity, and the curve
has the typical U form, the arms being of very unequal length.

O 20 4-0 60 30

VoliMne percentage of goUL.

Fig. 85. — Conductivity of silver-gold alloys.

/OO

This property of solid solutions is utilized in the construction
of electrical resistances, constantan, for instance, being an
alloy of 60 per cent, copper and 40 per cent, nickel. The
number of complete curves of this type which have been
determined is small, but the numerous partial determinations
which exist leave no doubt that the U form is characteristic
of all alloys forming an unbroken series of solid solutions,
whether the freezing-point curve has a minimum, as in the

memoir by W. Guertler, Zdtsc%. anorg. Chem., 1906, 61, 397, from wrhich
some of the data in this section have been taken.

256

METALLOGRAPHY

alloys of copper with gold or manganese, or is of the simpler
type exhibited by the gold-silver series.

The behaviour of alloys in which two series of solid
solutions are separated by a gap may now be inferred. Within
the limits of formation of the solid solutions, the depression of
conductivity takes place, as in alloys of gold and silver, very
rapidly, but between the limits of the saturated solutions the
variation is linear. The entire curve therefore consists of two
rapidly falling branches, connected by a straight line. If this

Fig

20 *0 60 ao 100

Volwrne. perceruape of cobalt.

86. — Conductivity of copper-cobalt alloys.

line happens to be nearly horizontal, it may be possible
to mistake the entire curve for the U form of completely
isomorphous metals. Exact measurements with a sufficient
number of alloys will, however, reveal the change of inclination
at the junction of the branches. Curves of this kind are given
by the alloys of copper and silver (Matthiessen) and of copper
and cobalt,^ the diagram of the latter being reproduced in
Fig. 86.

Inter-metallic compounds conduct like pure metals, although
their conductivity is always less than that of the better con-
ducting component, and in all instances hitherto observed falls

' G. Reichardt, Ann. Physik., igoi, [iv.] 6, 832. '

THE PHYSICAL PROPERTIES OF ALLOYS 257

below that calculated from the conductivity of the components
by the rule of mixtures. What has been said as to the
influence of compounds on the curve of hardness applies to a
considerable extent to the conductivity also. The presence of
a sharp cusp in the curve is unmistakable evidence of the
existence of a compound, but the converse does not hold good.
Should a single compound exist in the series and no solid

o 20 4-0 eo

Fig. 87. — Conductivity of copper -antimony alloys.

ao 100

Atom.xsb

solutions be formed, the curve consists of two intersecting
straight lines. A sharp cusp is most likely to occur if the
compound forms solid solutions with both components, as the
addition of either component, in accordance with the general
behaviour of solid solutions, then depresses the conductivity
very rapidly. The alloys of copper and antimony furnish a
typical example.^ The curve of conductivity (Fig. 87) has a

» G. Kamensky, PM. Mag., 1884, [v.] 17, 270. An equilibrium
digram, not, however, quite complete, has been determined by A. Baikoff,
Bull. Soc. d? Encouragement, 1903, 104, 626.

T.P.C. S

2S8 METALLOGRAPHY

sharp cusp, corresponding with the compound Cu^Sb, which
forms solid solutions with copper, whilst the second compound
CuaSb is only indicated by an abrupt change of conductivity,
in this case presenting itself as a minimum.

To sum up, although it is not in general possible to predict
the form of the conductivity curve of a series of alloys whose
structure is known, yet the observation of the conductivity
gives some very definite information as to the structure of the
alloys, in accordance with the following rules : —

1. A linear relation between conductivity and concentra-
tion indicates that the alloys are conglomerates, and that the
mutual solubility of the two metals in the solid state does not
exceed a very small amount, probably within o'l per cent.

2. A linear relation over the greater part of the range of
composition, the diagram being completed by two steeply
rising curves at the ends, indicates a limited mutual solubility
in the solid state, the extent of which may be determined from
the position of the points of intersection. If one metal dis-
solves the other to a small extent, but not conversely, the
rapid fall only takes place at one end of the diagram, giving
the L-shaped curve observed by Matthiessen in a number of
instances, of which the alloys of tin and bismuth may be
specially mentioned.

3. A smooth, U-shaped curve indicates the formation of a
continuous series of solid solutions. Careful determinations
are necessary to ascertain whether the curve is smooth
throughout; with an insufficient number of measurements a
short rectilinear branch, indicating a gap in the solid solutions,
may be overlooked.

4. A sharp cusp, with the point directed upwards, indi-
cates the presence of a compound capable of forming solid
solutions.

5. Abrupt changes of direction in the curve indicate the
appearance of a new solid phase at that point, but investiga-
tion by other means is required to determine the nature of the
phase.

When it is desired to study the influence of minute addi-
tions of one metal to another on its conductivity, there are

THE PHYSICAL PROPERTIES OF ALLOYS 259

advantages in plotting the specific resistance, instead of its
reciprocalj against the concentration by weight of the second
metal. Since the conductivity is most greatly modified . by
additions when the latter enter into solid solution, it is this
case which presents the greatest practical interest. For the
purposes of the electrical manufacturer, copper of the highest
attainable conductivity is required, and the depreciation of its
value by the presence of traces of impurities calls for chemical
and physical investigation. It has been shown that the curve
representing this depreciation falls very steeply at first. The
actual form is that of a hyperbola for at least the initial portion
of its course, and it follows that the curve of specific resistance,
which = I -T- specific conductivity, is very nearly rectilinear.

On comparing the influence of different elements on the
specific resistance of iron. Benedicks^ found that the increase
of resistance is proportional to the atomic percentage of the
added element, provided that the latter is present in solid solu-
tion. Carbon is held in solid solution only in hardened steels,
which are therefore taken for comparison." .In slowly cooled
steels the carbon is in the form of cementite, FesC, mechani-
cally mixed with metallic iron, and therefore only a small
increase of resistance is produced by the presence of small
amounts of carbon in this state. It appears possible, how-
ever, that the iron in slowly cooled steels retains up to 0*27
per cent, of carbon in solid solution, and within that limit the
linear atomic increase of resistance holds good. Benedicks
has adduced experimental evidence in favour of the existence
of such a solution in soft steels.^

For elements in solution in iron at the ordinary temperature
the relation —

W= 7-6 -I- 26-8 2C

gives the resistance in microhms per cc, SC being the total
dissolved carbon, -J- the sum of all the other dissolved elements,

' Zeitsch. fhysikal. Chem., 1902, 40, 545-

"^ Numerous data are found in the exhaustive researches of W. Barrett,
W. Brown, and R. A. Hadfield, Trans. Roy. Dubl. Soc, igo?, 8, i ;
Proc. Roy. Soc, 1902, 69, 480 j J. Inst. Elect. Eng., 1902, 31, 674.

» Thise four U Doctoral,, Upsala.

26o

METALLOGRAPHY

calculated to their " carbon-value " by the rule of atomic
equivalence given above. The table below contains the data
from a number of steels examined by Benedicks.

The samples consisted of Swedish steel of high quality.
Those marked q were hardened by quenching from about 800°,
except the last two, which were quenched from a yellow heat.
The measurements were made at 16°, and the resistances are
expressed in microhms per c.c. Calculations from the data
obtained by Barrett, Brown, and Hadfield," and Hopkinson,^
show that aluminium, chromium, and tungsten also obey the
same rule.

Carbide,
C

Dissolved elements.

Resistance.

No.

Harden-
ing C.

C-value
of Si.

C-value
ofMn.

20.

Obs.

Calc.

Diff.

I

_

o-o8

0-013

0-028

0-I2I

lo-s

10-8

-0-3

iq

—

o-o8

0'0I3

0-028

0-I2I

10-9

10-8

-fo-i

8

I '43

0-27

0-034

0-063

0-367

177

17-4

+0-3

7

1-23

d-27

0-051

0-063

0-384

17-9

17-9

0-0

4

0-63

0-27

o-iiS

0-089

0-477

20-2

20-4

-0-2

5

0-93

0-27

0-127

0-096

0-493

209

20-8

+0-1

6

ro8

0-27

o-iio

0-118

0-498

21-6

21-0

+0-6

2

o-i8

027

0-274

0-076

0-620

23-9

24-2

-0-3

3

0-28

0-27

0-363

0-096

0-729

27 -6

27-2

+04

2q

—

0-45

0-274

0-076

0-800

29-0

29-0

O'O

3q

—

0-55

0-363

0-096

1-009

34"4

34-6

-0-2

4q

—

090

o-ii8

0-089

1-107

36-9

37 '3

-0-4

sq

[o-H?]

I -20

0-127

0-096

1-423

[42-1]

457

[-3-6]

6q

—

1-35

o-iio

O-I18

1-578

49-6

49'9

-0-3

7q

1-50

0-051

0-063

1-614

50-6

50-8

— 0-2

The electrical conductivity of metals and alloys varies in a
marked degree with the temperature, and a general proportion-
ality between the conductivity and its temperature-coefiScient
was noticed by Matthiessen. This aspect has been more fully
studied by Guertler.' The conductivity decreases with rising
temperature, its coeflScient is therefore negative. The con-
ductivy of pure metals continues to increase as the temperature

' Loc. cit,

^ J. Hopkinson, Phil. Trans., 1885, 176, 463.

• Zdtsch. anorg. Chem., 1907, 61, 58.

THE PHYSICAL PROPERTIES OF ALLOYS 261

falls, and the course of the temperature curve has been taken
to indicate ' that at or near the absolute zero the conductivity
would become infinite. If the metal contains a second metal
in solution, its conductivity increases with falling temperature
until a finite value is reached, which remains constant at still
lower temperatures.^ This may be expressed by saying that
the additional resistance due to the presence of the added
metal is independent of temperature, and this rule holds good
of solid solutions at all temperatures.

For metallographic purposes it is convenient to express
temperature-coefficient as the percentage decrease of con-
ductivity between 0° and 100°, so that

P = ''°~/^°° .ioo

For pure metals P has the value 27-31, and the same number
is found for alloys which consist solely of conglomerates of their
components. If solid solutions are formed, the value of P
falls much lower. The curve representing the change of the
temperature-coefficient with concentration has, in fact, a very
similar form to the conductivity-concentration curve, with a
deep minimum in the middle of a series of solid solutions.
On this fact depends the possibility of obtaining alloys, the
conductivity of which is within certain limits independent of
the temperature. Thus constantan, an alloy containing 60 per
cent, copper and 40 per cent, nickel, has a zero temperature-
coefficient at ordinary temperatures, and the same is practically
true of the alloy containing 80 per cent, copper and 20 per cent,
manganese, and of platinoid (60 per cent. Cu, 24 per cent. Zn,
14 per cent. Ni, and 1-2 per cent. W). Guertler has used
three-dimensional diagrams to express the relations of the
conductivity, temperature, and concentration of alloys.

The value of P for inter-metallic compounds is somewhat

' J. A. Fleming and J. Dewar, Phil. Mag., 1892, [v.] 84, 326; 1893,
[v.] 36. 271.

* A summary of the facts relating to the conductivity of metals and
alloys at low temperatures is given by J, Clay, Jahrb, Radioakt. Elek-
tronik, 191 1, 8, 383.

262 METALLOGRAPHY

smaller than for pure metals, being 26'8 for AgsSn, 22 for CuSn,
and 2 2 '4 for AuSnj, so that their inversion temperature probably
lies above that of the pure metals. The presence of compounds
affects the temperature-coefficient diagram in the same way as
the conductivity diagram. When the practical difficulties are
such that it is not possible to obtain a complete series of alloys
in a sufficiently homogeneous and non-porous state for con-
ductivity measurements, P may be advantageously determined
by observations of the conductivity at 0° and 100°, and the
same conclusions as to the constitution of the alloys may be
drawn as from the conductivity curve.

An abrupt change in the conductivity occurs at the melting-
point. There is a remarkable similarity between the conduc-
tivity curves of liquid and solid alloys. Thus the conductivity
of molten alloys of lead and tin is proportional to their
composition, whilst alloys of copper and nickel give a U-shaped
curve. Inter-metallic compounds which are sufficiently stable
to persist in the molten state cause the appearance of a peak
or kink in the curve. Examples of this arc the compounds
HgaNa, HgaK, and CujSb.^

Lastly, mention may be made of the ratio of thermal to
electrical conductivity. Whilst the thermal conductivity, X,
has been little employed as a means of studying alloys, its ratio

to the electrical conductivity, -, has some value for this purpose.

The values of - for pure metals at a given temperature are
nearly equal, and increase proportionally to the absolute
temperature. The values of ( - ) ; ( - ) vary, for different
metals, only between i'i2 and i'35.^

In alloys, - has a higher value than in pure metals if solid

K

, ' P. Miiller, Mdallurgie, 1910, 7, 730, 755 ; K. Bornemann and G. von
Rauschenplat, ibid., 1912, 9, 473, 505.

^ G. Wiedemann and R. Franz, Ann. Physik., 1853, [li.] 89, 497 ; W.
Jaeger and H. Diesselhorst, Abh. phys.-tcchn. Reichs-Anst., 1900, 3, 269;
E. Griineisen, Ann. Physik., 1900, [iv.] 3, 43.

THE PHYSICAL PROPERTIES OF ALLOYS 263

solutions are formed, that is, the formation of the solution
depresses the thermal relatively less than the electrical con-
ductivity, - being, for instance, 665 for copper, 699 for nickel,
and 1 1 06 for constantan. If solid solutions are not formed,
- has the same value as for pure metals.-* The reverse action,

a diminution of the ratio, has been observed to occur in the
alloys of copper with arsenic and phosphorus." It remains to
be seen whether this influence of non-metals is a general one.''
The practical methods of determination of A. are fully
described in the memoir of Jaeger and Diesselhorst {loc. cit.).

Thermo-electric Power

Alloys, like pure metals, develop a thermo-electromotive
force when two dissimilar specimens are connected, one junction
being heated or cooled to a different temperature from that at
which the other is maintained. In making the experiments
the two specimens are only in contact with one another at a
single junction, their other ends being connected with copper
wires leading to the galvanometer or potentiometer employed
to measure the electromotive force. When alloys are under
investigation, it is usual to employ a junction consisting of the
alloy and of one of its component metals, although another
metal, such as copper or platinum, may also be used as a,
standard. The specimens are most conveniently taken in the
form of rods or wires, but this is not always practicable with
brittle alloys. Experiments on crystals of bismuth* indicate
that the thermo-electric power varies with the orientation, so
that the results obtained with cast or drawn rods or wires are
average values. The specimens should be thoroughly annealed,

> F. A. Schulze, Ann. Pkysik., 1902, [iv.] 9, S5S-
2 A. Rielzsch, ibid., 403.

» See also E. van Aubel and R. Paillot, J. Physique, 1895, ["i-I
4, 522.

< L. Periot, Arch. Sci. phys. not., 1898, [iv.] 6, 105, 229 ; 1899, [iv.]

7, 149.

264 METALLOGRAPHY

as mechanical work has a great influence on the thermo-electric
properties.

Many investigators have found that alloys do not necessarily
occupy a position in the thermo-electric series intermediate
between those of the component metals. The data available
are insufficient to determine the cause of the variations
observed, but it is probable that the formation of inter-metallic
compounds is an important factor. For example, whilst the
amalgams of Zn, Sn, Pb, Cd, and Cu fall between mercury and
the corresponding metal, the amalgams of Bi, Tl, Mg, and Na
deviate from this rule. The former series contains metals
which do not combine with mercury, whilst the latter series
consists of metals which form compounds.

The E.M.F. developed in a thermo electric circuit varies
greatly with the temperature, and for most pairs of metals or
alloys becomes zero at a certain temperature, the neutral or
inversion temperature, after passing which it changes in sign.
For a large number of pairs, the relation is expressed by the
formula —

£ = (/,- A) \a -f c{h + A)] '

in which t-^ and 4 are the temperatures of the junctions, and
a and c are constants. The inversion temperature is given by
the formula —

_ _ a

2C

The validity of Avenarius' equation has been tested for a
large number of metals and alloys,^ and more complicated
expressions have been introduced to cover the observed
deviations. Experiments in which the junction is kept at the
low temperature of liquid air have been made,' and further
deviations from the simple relation have been found.

' R. Avenarius, Ann. Physik., 1863, 119; 4o6; 1864, 122, 193; 1873,
149, 372.

^ W. Jaeger and H. Diesselhorst, Abh. pliys. techn. Reichs-Anst., 1900,
3, 269 ; L. Holborn and A. Day, Sitzmigsber. k. Akad. Wiss. Berlin,
1899, 691.

' J. Dewar and J. A. Fleming, Phil. Mag., 1895, [v-] 40, 95 ; De Metz,
Compt. rend., 1904, 139, 447.

THE PHYSICAL PROPERTIES OF ALLOYS 265

The variation of the thermo-electric power with the compo-
sition of alloys follows very similar laws to ?he conductivity.
Inter-metallic compounds are commonly marked by a peak on
the curve, and the indications are often even more distinct
than those of the conductivity or its temperature-coefficient.
Excellent examples are furnished by the alloys of magnesium
with aluminium and with silver/ and of tellurium with bismuth
and with antimony.^

The measurement of the thermo-electric force when coupled
with an inactive metal, such as platinum, may also be employed
as a means of locating transformations in an alloy, a critical
point being indicated by an abrupt discontinuity in the E.M.F.
curve. This method has often been applied to steels, and has
yielded results confirmatory of those obtained by other methods.
The use of thermo-electric measurements in detecting the effect
of mechanical changes on metals and alloys will be referred to
later (Chapter XVI.).

Magnetic Properties

Magnetic properties are exhibited by the majority of metals
and alloys in so slight a degree that their measurement is with-
out metallographic interest. It has been shown' that the
magnetic susceptibility of the elements is a periodic function
of the atomic weight, but it is only in a few groups that the
value exceeds a very minute amount. The group Fe, Ni, Co,
of the so-called ferromagnetic metals, is distinguished from all
others by the highly magnetic character of its members, and
until recently the interest of the metallurgist in magnetic
properties was confined to these metals and to certain of their
alloys. The discovery in 1903 of a strongly magnetic alloy

' W. BroniewsW, Compt. rend., 1910, 150, 1754; 1911, 153, 85.

' W. Haken, Ann. Physik., 1910, [iv.] 33, .291.

3 J, Koenigsberger, Ann. Physik., 1898, [iii.] 68, 698 ; Stefan Meyer,
ibid., 1899, [iii.] 68, 324 ; Monatsh., 1899, 30, 369, 797 ; Ber., 1900, 33,
1918; O. Liebknecht and A. P. Wills, Ann. Physik., 1900, [iv.] 1, 178;
H. du Bois and O. Liebknecht, Ber., 1899, 32, 3344 ; H. du Bois, Rapp.
Congr. intern. Phys., Paris, 1900, ii. 460.

266

METALLOGRAPHY

of copper, manganese, and aluminium,' has given a new
stimulus to the study of the magnetic properties of alloys, since
it is now clear that metals, only feebly magnetic in themselves,
may under certain conditions form magnetic compounds with
one another.

The change of magnetic properties with temperature has
also great metallographic importance. The change is not
continuous, but takes place abruptly at certain critical tempe-
ratures, and it becomes of interest to correlate these critical
points with the discontinuities in the thermal, microscopical, and
other properties. Much research has been directed, in particular,
to the changes which occur at the low temperature of liquid air.

The phenomena of magnetism are so complex that it is
impossible to give in this place more than the briefest outline
of the magnetic study of alloys, and reference must be made to
text-books on physics for details as to the methods employed
and the results obtained.

For the purpose of metallographic investigations, the
specimens are most conveniently examined with the aid of a

F
L °

E

ffl A

C

^ 1

m

III ^mm4

1

D

^

Fig. 88. — Gray and Ross's magnetometer.

magnetometer, the deflections of a minute suspended magnet

when placed near to the specimen being observed. Such

magnetometers take many forms. Since the metallographist

may desire to make observations on the same specimen at

many different temperatures, all other conditions remaining

identical throughout, this requirement must be kept in mind in

the design of an instrument. Gray and Ross's magnetometer

is simple, and lends itself readily to accurate investigations of

' F. Heusler, VerA. deut. physikal. Ces., 1903, 219 ; F. Heusler, W.
Stark, and E. Haupt, ibid., 222.

THE PHYSICAL PROPERTIES OF ALLOYS 267

this kind.^ This instrument is shown in plan in Fig. 88.
The heavy cross-shaped base is of mahogany, with a channel
in which the blocks supporting the coils, etc., slide and are
clamped by means of friction clamps. The solenoid A is of
insulated wire wound on a thin brass water-jacket, which keeps
the wire cool even when the heating furnace is introduced.
The magnetometer E consists of a small magnet, 8 mm. in
length, with a concave mirror attached, suspended by a quartz
fibre. The incandescent lamp L has a fine cross-wire, the
image of which is reflected by the magnetometer mirror on to
the scale S. The coil B, which is wound in sections, is intended
to compensate for the effect of the current in the solenoid. As
a small displacement of B has a considerable effect on the
deflection of the needle, and it is difficult to secure a suffici-
ently exact adjustment, a second compensating coil, C, is placed
at a greater distance, and on the opposite side of the. solenoid.
Since C contributes only a small fraction of the balance, it is
easily adjusted to the proper position. The small coil D
compensates for any deviation from the coaxial arrangement
of the other parts of the apparatus. Lastly, a coil, G, connected
with a cell and reversing key, is added as a means of bringing
the needle rapidly to rest.

The instrument is placed with its principal axis exactly in
an east-and-west direction. The coils are adjusted until no
deflection of the needle is produced when the current in the
solenoid is reversed, even when a small permanent magnet is
placed on the transverse arm, as at F.

The width of the water-jacket of the solenoid is sufficient
to admit the electric furnace, a porcelain tube wound with
platinum wire packed in kaolin, and enclosed in an outer tube
of Jena glass. For experiments at low temperatures, a glass
tube or a Dewar vessel containing liquid air is used. The
specimens are used in the form of rods, 20 cm. in length and
I cm. or less in diameter.

The field strength H is calculated from the strength of the
magnetizing current in amperes C, being equal, with a very
small correction, to o"4ir;/C, where n is the number of turns in

' J. G. Gray and A. D. Ross, Proc. Roy. Soc. Edin., 1909, 29, 182.

268 METALLOGRAPHY

__ the solenoid per unit length. Field strengths up to 400 units
may be obtained without difficulty with the above apparatus.
The magnetic moment I is measured by the deflection of the
magnetometer needle. The ratio I/H gives the susceptibility k.

Many other forms of apparatus are employed in magnetic
investigations. Thus curves of magnetization and hysteresis
measurements are commonly made with annular specimens of
metal, on which coils for magnetization and for measurement of
the magnetic induction are wound.^ The induction is measured
by a ballistic galvanometer. For rapid determinations of the
hysteresis for technical purposes, instruments are used in which
the specimen is rapidly rotated between the poles of a curved
suspended permanent magnet, the hysteresis being measured
by the torque produced, tending to rotate the magnet.* The
magnetic balance, a modification of the older instrument of
Hughes, has also found technical application in various forms.'

The magnetometric method is, however, the simplest in
character, and the most convenient for the scientific
investigation of alloys.

In the study of alloys, the susceptibility, k, or the ratio of the
magnetic moment, I, per c.c. to the strength of field, H, is the
most convenient magnitude to employ. The permeability,
/«. = I + 47rK, is much used in considering metals and alloys
from the point of view of their technical application.

When pure soft iron is magnetized by an external field of
gradually increasing strength, the value of k is at first almost
constant so long as H does not exceed 0-05 C.G.S. unit.* For
very pure iron, k = about 30,^ but the value always falls below
this for ordinary samples. As H rises to 10, the susceptibility
increases rapidly to a maximum of about 400. For higher

■ J. A. Ewing and H. G. Klaassen, Fhil. Trans., 1893, 184, A, 985 ;
Mme. M. S. Curie, Bull. Soc. d' Encouragement, 1898, 36 ; Etude des
Alliages, 1901, 159.

^ J. A. Ewing, ibid.

' ]. A. Ewing, y. Inst. Elect. Eng., 1898, 27, 526 ; de Kryloff, Rev.
de Metallurgie, 1 905, 2, 425.

■■ For the dimensions of the units employed, reference should be made
to works on physics.

' H. du Bois, Rapp. Congr. intern. Phys., 1901, ii, 460.

THE PHYSICAL PROPERTIES OF ALLOYS 269

values of H the susceptibility decreases, that is, a much greater in-
crease of H is necessary to produce a given increase in I than in
the earlier stages. After the magnetization has reached two-thirds
of its maximum value, it increases only very slowly, the suscepti-
bility therefore diminishing also, to become zero when H = 00 .
The strength of field may be carried to 20,000 units without
great difficulty, and a value of H = 51,600 has been attained.'

When the maximum value of I has been reached, the
metal is said to be magnetized to saturation. This point lies
in the case of iron near I = 1850, but is lower in the case of
other magnetic substances, being 580 or less for nickel and
1370 for cobalt.

The magnetic properties of metals and alloys are influenced
in a marked degree by change of temperature. A complete
representation of the magnetic behaviour of a substance under
conditions of equilibrium is therefore only possible by means
of a three-dimensional model, in which I is shown as a function
of the strength of field H and the temperature B. In practice,
we may use plane sections of two kinds through the model,
one series representing the variation of I with H at constant
temperature, and the other the dependence of I on 6 under the
influence of a constant field.

Considering first the case of iron, the effect of heating
above the ordinary temperature is at first to increase the
susceptibility, and at the same time to diminish the value of I
at which the susceptibility is a maximum. The increase of k
just before reaching 760° is extremely rapid, but at this point
the magnetization falls rapidly. Although it is usual to
describe this temperature as a critical point, it is more correct
to speak of a transformation range, as the change from a
highly magnetic to a practically non-magnetic modification of
iron is not instantaneous, but continues over a definite range
of temperature. It corresponds with the a— j.jS change, which
appears also on the expansion curves as a gradual transforma-
tion, although it is more distinctly indicated at 760° on the
thermal curves. The reappearance of magnetic properties on
cooling from a high temperature takes place over the same
* E. T. Jones, Ann, Physik., 1896, "[iii.] 57, 273.

270 METALLOGRAPHY

temperature range. At higher temperatures, further variations
in the magnetic properties of iron have been observed,' includ-
ing a sudden and very large increase in the susceptibility at
1 280° ; but it is at present impossible to correlate these changes
with any known discontinuities in the other properties of iron,
and until the experiments have been repeated with all the
precautions necessary to avoid the disturbing influence of
dissolved gases, etc., the results must be accepted with reserve.
Very numerous observations of the magnetic behaviour of iron
at low temperatures, down to —186°, have been made, the
results indicating that the saturation capacity increases with
diminishing temperature. Different specimens of iron and steely
however, behave very differently, as regards the changes of
susceptibility at low tempertures.^ Curie's law, which holds
good for paramagnetic substances, that the susceptibility is pro-
portional to the absolute temperature, is apparently not fulfilled
for the strongly ferromagnetic metals.

The magnetic observations forming part of a metallographic
investigation have been confined, in the "majority of cases, to
the determination of the temperatures at which marked
magnetic properties disappear on heating and reappear on
cooling. The magnetic measurements are often merely
qualitative, the point being noted at which a poised magnetized
needle is attracted. If we are dealing with alloys of iron, the
temperature of transformation is the same throughout all alloys
of the series in which iron is present as a separate phase. Alloys
containing iron in a solid solution exhibit a depression or
elevation of the traHsformation point, according to the con-
centration of the solid solution. The same statement holds
good of cobalt and nickel, the transformation temperatures of
which are 1159° and 320° respectively.

A simple instance of the behaviour of solid solutions of
magnetic metals is afforded by the alloys of nickel and cobalt,^
which form an isomorphous series. The complete diagram

• P. Curie, Ami. Chim. Phys., 1895, [vii.] 5, 289; see also D. K.
Morris, Phil. Mag., 1897, [v.] 44, 213.

2 J. A. Fleming and J. Dewar, Proc. Roy. Soc, 1896, 60, 81.

' W. Guertler and G. Tammann, Zeitsch. anorg. Chem., 1904, 42, 353.

THE PHYSICAL PROPERTIES OF ALLOYS 271

has the form shown in Fig. 24, the magnetic transformation
being represented by a continuous pair of curves between 320°
and 1159°, the space between which represents the interval
over which the transformation takes place. There is thus com-
plete isomorphism between /3-nickel and /3-cobalt, and also
between d-nickel and a-cobalt.

The alloys of iron and manganese exhibit a different
behaviour. The magnetic transformation temperature of iron
is rapidly lowered by small additions of manganese, and the
diagram, so far as it has been investigated, resembles Fig. 25.
These alloys may be obtained in a partly magnetic state by
quenching from above the transformation curve.' Aluminium
also depresses the transformation temperature of iron.

As a type of alloys in which the magnetic metal persists
throughout as an independent phase, we may take the series
gold-nickel, the freezing-point curve of which has the simple
V form, solid solutions being absent. The transformation
temperature of nickel remains constant thoughout the series,
and the boundary separating the regions of stability of a and /8
nickel therefore runs horizontally across the diagram.^

Very remarkable conditions are presented by the alloys of
iron and nickel. The freezing-point curve indicates that
soUd solutions are formed, as well as a compound, the
formula of which is provisionally written as FCaNi.^ The
magnetic transformation temperature of iron is lowered by
the addition of nickel, and that of nickel raised by the addition
of iron, but the curve connecting these two points, instead of
being continuous, rises to a maximum at 600° and 70 per cent.
Ni, with an intermediate minimum or eutectoid point below 0° at
25 per cent. Ni.* On the iron side of this point, that is to say, in
alloys containing less than 25 per cent. Ni, there are two trans-

^ ' R. A, Hadfield, Proc. Inst. Civ. Eng., 1888, 93, iii. i ; M. S. Curie,
Ettule des Alliages, ill. ^'^'^ A^ freezing-point curve, see M. Levin and
G. Tammann, Zeitsch. anorg. Chem., 1905, 47, 136.

= M. Levin, ibid., 1905, 46, 238.

» W. Guertler and G. Tammann, ibid., 1905, 45, 205 ; R. Ruer,
Metallurgie, 1909, 6, 679.

< F. Osmond, Comp. rend., 1B94, 118, S32 ; 1899, 128, 304, 1396 j
F. Osmond and G, Cartaud, Rev, de MHallurgie, 1904, 1, 69,

272 METALLOGRAPHY

formatiun curves, so that each alloy of the group has two critical
temperatures, 4 and A- The lower (/j) curve falls much more
steeply than the upper (4) curve. Between these curves each
alloy is capable of existing in two conditions, a magnetic and
a non-magnetic. A magnetic alloy loses its magnetism on
heating at 4i whilst an alloy cooling from a high temperature
remains non-magnetic until 4 is reached. In other words, the
transformation is overstepped in«both directions, the region
lying between the two curves increasing in breadth with in-
creasing nickel content, until the range within which both the
magnetic and non-magnetic alloys may exist amounts to 600°
when the nickel reaches 25 per cent. Beyond this limit, the
transformation becomes almost exactly reversible, and the curve
with a maximum at 70 per cent. Ni represents the change both
on heating and on cooling with fair approximation. The nature
of the metastable conditions in the alloys of nickel and iron
will be discussed later, in connection with the structure of the
meteoric irons (p. 383).^

A very interesting group of magnetic alloys consists of
ternary alloys of manganese, often referred to as Heusler's
alloys. It was observed in 1892'' that whilst ferro-manganese
and ferro-aluminium are non-magnetic, ternary alloys containing
only 10-14 per cent, of iron, the remainder being manganese
and aluminium, are strongly magnetic, some members of the
series being comparable with iron itself. The observation was
subsequently made^that alloys of copper and manganesebecome

' For the magnetism of the nickel-iron alloys, see J. Hopkinson, Proc.
Roy. Soc, 1890, 47, 23 ; C. E. Guillaume, Compt. rend., 1897, 134, 176,
1515; 125, 23s ; 1898, 126, 738; Etude des AlUages, 459; L. Dumas,
Compt. rend., 1900, 130, 357 ; E. Dumont, ibid., 1898, 126, 741 ; H.
Tomlinson, Proc. Roy. Soc, 1884, 56, 103.

^ T. W. Hogg, Chem. News, 1892, 66, 140.

' See F. Heusler, Verh. deut. physikal. Ges., 1903, 5, 219 ; W, Stark
and E. Haupt, ibid., 222 ; E. Take, Verk. deut. physikal. Ges., 1905, 7,
133 ; Attn. Physik., 1906, [iv.] 20, 849 ; P. Asteroth, Verh. deut. physikal.
Ges., 1908, 10, 21 ; H. Fassbender, ibid., 256 ; F. Heusler and F. Richarz,
Zeitsch. anorg. Chem., 1901, 61, 265 ; J. A. Fleming and R. A. Hadfield,
Proc. Roy. Soc, 1905, 76a, 271 ; A. Gray, ibid., 1906, 77a, 256 ; A. D.
Ross, Proc. Roy. Soc. Edin., 1907, 27, 88 ; A. D. Ross and R. C. Gray,
ibid., 1909, 29, 274 ; A. A. Knowlton, Phys. Rev., 1910, 30, 123 ; 191 1,
32, 54. A general discussion of the subject, including papers by various
authors, is to be found in Trans. Faraday Soc, 12 12, 8.

THE PHYSICAL PROPERTIES OF ALLOYS 273

magnetic when aluminium, tin, antimony, bismuth, arsenic, or
boron are added in certain proportions. The copper-alu-
minium-manganese alloys have been most fully investigated,
and are found to behave in every respect as ferromagnetic
substances. Quenching from a high temperature frequently
renders the alloys non-magnetic, their magnetic properties
being developed by heating to no" or 140°.

It is evident from the variabihty of the magnetic properties
that we have to deal with a system having a great tendency to
assume a metastable condition. Take has found that the
transformation temperature is raised by repeated heatings.
The cooling curves of the alloys are marked by several
arrests, some of which are accompanied by changes of volume,
indicating that the reactions in solid solution are of a complex
character.

Copper and manganese are isomorphous, whilst aluminium
forms compounds with both, amongst the compounds which
have been definitely recognized being CujAl and MnjAl.
Plotting the results hitherto obtained in this ternary series on
a triangular diagram, it is found that the most strongly magnetic
members of the series falf on a line connecting these two
compounds, and are therefore to be considered as mixtures,
probably isomorphous, of CujAl and MuaAl. The appear-
ance of magnetic properties is certainly connected with the
formation of these inter-metallic compounds, and is not to be
attributed, as was at first supposed, merely to a displacement
of a transformation point of manganese by alloying with other
metals.

Several other compounds of manganese, especially MnB,
MnSb, MnAs, Mn^Sn, and MnBi, are strongly magnetic'
The behaviour of the bismuthide is remarkable, in view of
the fact that bismuth itself is diamagnetic, that is, its suscepti-
bility has a negative value. The relation of magnetic properties
to constitution has been determined for several series of alloys
by Honda.^

' See the papers of F. Heusler, quoted on p. 272, and also E. Wede-
kind, Ber., 1907, 40, 1259 ; Zeitsch. physikal. Ckem., 1909, 66, 614 ;
E. Wedekind and T. Veit, Ber., igii, 44, 2663.

^ K. Honda, Ann. Pfiysik., 1910, [iv] 32, 1003. For the magnetic
T.P.C. T

274 METALLOGRAPHY

Hysteresis and Coercive Force

When a metal or alloy is submitted to the action of an
alternating magnetizing field, a certain quantity of energy is
dissipated in each cycle by conversion to heat. This
quantity, represented graphically by the area of the correspond-
ing loop in the I-H curve, obtained after the material has
been brought into a steady state by several repetitions of the
cycle, is called the hysteresis, and its amount governs the
applicability of the material in electrical constructions. Trans-
former iron, for example, which has to pass through rapidly
repeated magnetic cycles, must be a variety exhibiting the
smallest possible hysteresis, in order to avoid undue heating
of the metal and loss of energy. The interval between the
transformation temperatures on heating and on cooling in the
case of such materials as the irreversible nickel steels (p. 272)
is sometimes called the temperature hysteresis, but must not be
confused with the quantity just described.

As the hysteresis evidently represents the energy absorbed
in twice reversing the polarized position of the molecules of
the material," it has a great importance in connection with
molecular theories of magnetism.^ Soft, pure iron has the
lowest hysteresis of any known ferromagnetic substance.
Hardening by mechanical work has the effect of greatly in-
creasing the hysteresis, whilst the addition of other elements,
especially carbon, as a rule, also increases it. It is true that
certain alloys of iron, in particular those containing small
quantities of aluminium, have been found to have even a
lower hysteresis than the purest specimens of iron ; but it is
probable that the greater part of the effect is due to the action
of the added metal in removing oxygen, which is generally
present in iron.

propetties of a very extensive series of alloys of iron, see the papers by
Barrett, Brown, and Hadfield, cited on p. 259.

' J. A. Ewing, Proc. Roy. Soc, 1S90, 48, 342.

* See, for a review of the subject, E. Warburg, Rapp. Congr. intern,
Phys., 1900, ii. 509.

THE PHYSICAL PROPERTIES OF ALLOYS 275

The hysteresis of the Heusler alloys presents a number of
highly interesting features, the phenomena having been as yet
very incompletely explained. Reference must be made to the
papers quoted on p. 272 for details.

In the construction of permanent magnets, the properties
required are the reverse of those demanded in transformer iron.
The coercive force of such materials is said to be large. The
coercive force of iron is greatly increased by the presence of
carbon and other elements held in solid solution, hence per-
manent magnets are made of steel hardened by quenching.
Even pure iron has an increased coercive force after quenching,'
possibly due to internal strain. Tungsten and molybdenum are
particularly effectual in increasing the coercive force. It is
evident that in all future investigations of this very important
subject, attention should be paid to the relations between the
heat treatment of the materials examined and the equilibrium
diagram of the alloys as derived from thermal and micrographic
observations.

' E. Maurer, Metallurpe, 1909, 6, 33.

CHAPTER XIII

ELECTROMOTIVE FORCE AND CORROSION

The electric potential assumed by an alloy in contact with an
electrolyte is a property of high diagnostic value in the study
of constitution, and at the same time has great practical
importance as determining the liability of the alloy to corrosion
when exposed to the influence of electrolytes alone or in
contact with other metals. The experimental determination
of the potential is complicated by the fact that alloys, not
being elementary substances, are liable to react with the elec-
trolyte in such a way as to bring about a change of composition
at the surface of the electrode^ causing the potential observed
to vary with the time. Further, if the alloy be heterogeneous
in structure, local electrolytic actions take place between the
micrographic constituents at the surface, again resulting in an
alteration of the effective composition of the alloy, and conse-
quently in a variation of the potential. A third difficulty arises
from the necessity of finding a suitable electrolyte with which
the alloy can be in equilibrium. Whilst it is sufficient to
examine a pure metal in contact with a solution of one of its
own salts, a complex electrolyte containing definite proportions
of salts of the component metals is required in the case of
alloys.

It is to this last condition, so commonly overlooked, that
the uncertainty of the results obtained by most of the earlier
investigators is due. With a view to the practical study of
corrosion, it was usual to compare the electromotive force
developed by various alloys when connected with some
standard metal such as copper, in a solution of an alkaline
salt, or of an acid, or in sea-water.

276

ELECTROMOTIVE FORCE AND CORROSION 277

The earliest determinations of a more scientific character
are due to Laurie," who employed a solution of a salt of one of
the component metals as the electrolyte, and made efforts to
avoid polarization. Thus alloys of copper were compared
with copper in a solution of cuprous iodide, copper-tin alloys
in stannous chloride compared with copper in cupric sulphate,
separated by a porous partition, and so on. Only relative
values were obtained in this way, as account was not taken of
the concentration of the electrolyte j but marked discontinuities
in the KM.F.-composition curves were observed in several
cases, the copper-tin alloys, for instance, exhibiting a sudden
change of potential at the composition corresponding with the
formula CujSn.

In his numerous series of measurements, Herschkowitsch '
employed as electrolyte a normal solution of a salt of the
more positive metal, the comparison electrode being a rod of
the less positive metal. Characteristic curves were obtained
for a large number of alloys, the discontinuities being very
clearly marked in many instances. Whilst, however, this
method of working is satisfactory when the two metals com-
posing the alloy differ widely in their position in the electro-
chemical series, it fails in other cases, as the electrolyte used
is not one with which a true equilibrium is possible. The con-
ditions of equilibrium have been fully investigated from the
point of view of the phase rule,' and the following are the
principal conclusions that have been reached.

I. The two metals do not form either compounds or solid
solutions.

The potential difference between a metal, Mj, and a solution
containing only a salt, MiZ, is

El = o-Zbo^l,7-^ X 10-* . . . . (i)

where «i is the valency of the metal, Pi its solution pressure,

' A. P. Laurie, Trans. Chem. Soc, 1888, 53, 104 ; 1889, 55, 677 ;
1894, 65, 1031 ; Phil. Mag-., 1892, [v.] 33, 94; Zdtsch. physikal. Chcm.,
1909, 67, 627.

'' M. Herschkowitsch, Zeitsch. physikal. Chem., 1898, 37, 123.

* W. Reinders, ibid., 1903, 42, 225.

278 METALLOGRAPHY

and/i the ionic concentration of the metal in the electrolyte;
o-86o X IO-* is the " electrolytic gas constant/' R/F ; T is the
absolute temperature.' If some of the Mi ions are replaced by
M2, px becomes smaller, and Ei is increased. The potential
difference between the second metal, Ms, and a solution of its
salt is similarly altered by the presence of Mj. The two
logarithmic curves intersect at a point at which Ej = Ej, so
that

-/3=-/«?^ (2)

or, if «i = «2, Pi : Pa = /i : A) or the ratio of the ionic con-
centrations under conditions of equilibrium is equal to the ratio
of the solution pressures. If these differ very widely, as in the
case of copper and zinc, /a becomes very small, and a mere
trace of the metal Mj is sufficient for equilibrium ; Herschko-
witsch's procedure is therefore justified. It is evident, however,
that when Pj and Pa are approximately equal, as for silver
and mercury, a mixed electrolyte of suitable composition is
necessifiry.

II. The two metals form a homogeneous solid solution (the
same reasoning applies also to liquid amalgams).

Equation (2) applies to the state of equilibrium if Pj and P2
are now taken to represent the partial solution pressures of Mi
and Mj respectively. The solution pressure of Mj is lowered
by the presence of Ma in solid solution, and for small concen-
trations the lowering may be safely assumed to be proportional
to the molecular concentration of Ma in the electrode. If this
concentration = x, then P/ = Pi(i — x). Pa' = K^, where K
is the solution pressure of Ma in the presence of Mi, and is
probably rather less than Pa. Then

or, if «i = «2,

n /i «a P:

/a _ K « , .

A-P? i-~~x ('^^

' W. Nernst, Zeitsch. physikal. C/iem., 1889, 4, 129.

ELECTROMOTIVE FORCE AND CORROSION 275

or the ratio of the two metallic ions in the electrolyte is to the
molecular ratio of the two atoms in the alloy as K : P,'.

As before, if K and P/ differ very widely, the electrolyte
will contain almost exclusively ions of the more positive metal,
whatever may be the concentration of that metal in the elec-
trode. The E.M.F. is thus a logarithmic function of x. In

Fig. 89 the full curve represents the variation of E with x, the

dotted curve that of E with -^

A+ A
If the two metals form two series of solid solutions
separated by a gap, the two saturated solid solutions are in
equilibrium with one another and with the electrolyte. Within
the limits of composition represented by the gap, the E.M.F.
is therefore constant. Such a condition is presented by the
cadmium amalgams, the E.M.F. curves of which, for three
different temperatures, are shown in Fig. 90.' The electrolyte
used was a solution of cadmium sulphate. The width of the
gap is seen to diminish as the temperature rises; and these
E.M.F. measurements were, in fact, employed to determine
the slope of the lines DP and EQ in Fig. 21 (p. 54). On
each E.M.F. curve, the first horizontal portion represents the
interval between the liquidus and solidus curves, and the
second the interval between the lines DP and EQ.

' H. C. Bijl, Zeitsch. physikal. C/iem., 1902, 42, 641.

28o

METALLOGRAPHY

III, The two metals form a compound.

The problem of determining the E.M.F. is now complicated
by the necessity of making an assumption as to the nature of
the ions sent out by the compound. Reinders, making the
assumption that it sends out ions of the same composition as
the compound, concludes that the E.M.F. is a maximum when
the ratio of the ionic concentrations = the atomic ratio of the
metals in combination.

Taking the simplest case, that of a series in which a single
compound occurs and solid solutions are not formed, as in the

O JO 20 30 4-0 SO 60 70 SO SO 100

AtoTruc/oed

Fig. 90. — E.M.F. curves of cadmium amalgams at 25°, 50°, and 75°.

series magnesium-lead, magnesium-tin, or magnesium-bismuth,
the E.M.F. curve has the form shown in Fig. 91. The
potential is that of the more positive metal, so long as any of
the latter is present as a distinct phase. At the composition
of the compound, the potential suddenly falls to that of the
compound or of the second metal, whichever has the lower
solution pressure. If solid solutions are formed, or if the
metals form several compounds, the curves obtained present a
combination of the characters described. Thus the E.M.F. of
thallium amalgams, measured against a mercury electrode,
increases in a regular manner from practically zero for pure

ELECTROMOTIVE FORCE AND CORROSION 281

mercury^ to a constant value at 33-3 atomic per cent, at 0°,
indicating that amalgams containing less than that proportion
of thallium consist of solid solutions of the compound HgjTl
in mercury, whilst amalgams richer in thallium consist of
conglomerates of crystals of Hg^Tl and thallium.^

If the compound forms solid solutions with both its com-
ponents, its composition is not indicated by a discontinuity in
the E.M.F. curve at that point. In Fig. 92 a system, the
freezing-point curve of which is indicated in the upper diagram,

Mz

M,Mi

M,

Atomic percentage 'of M}
Fig. 91.

is likely to give an E.M.F. curve like that represented in the
lower diagram. On the supposition that a discontinuity indi-
cates the formation of a compound, the composition of the
latter would probably be put at A rather than at B, and a
wrong formula would in this way be assigned to the compound.
A very numerous series of recent determinations are due to
Pushin and his collaborators,' who, finding the application of
the rules for the composition of the electrolyte given above
to present certain difficulties, have usually employed acids

' Not exactly zero, as the one electrode is in contact with an electrolyte
consisting principally of thalloiis chloride, whilst the other is in contact
with niercurous chloride and potassium chloride (calomel electrode).

^ A. Sucheni, Zeitsch. Elektrochem., 1906, 12, 726.

' N. A. Pushin, J. Russ. Pkys. Chem. Soc, 1907, 39, i. 13, 353, 528,
86g ; Zeitsch. anorg. Chem., 1907, 56, i ; N. A. Pushin and N. P.
Pashsky, J. Russ, Phys. Chem. Soc, 1908, 4.0, S26 ; N. A. Pushin and
P. N. Laschtschenko, ibid., 1909, 41, 23; Zeitsch. atwrg. Chem., 1909,
G2, 34.

282

METALLOGRAPHY

or alkalis forming sparingly soluble salts with the more
positive metal, a simple salt of the more positive metal
being used when the solution pressures differ widely. Thus
the potential of the alloys of lead with the platinum metals

M2

\' /

C

A

Ma

M/

Atomic percentage of M2
Fig. 92.— Constitution and E.M.F.

was measured in a normal solution of lead nitrate against
lead. Calcium hydroxide was found to be the most suit-
able electrolyte for aluminium alloys and potassium hydroxide
for the alloys of lead and tin. Several typical curves selected
from Pushin's results are collected in Fig. 93. Anti-
mony and bismuth, being isomorphous, give a continuous

ELECTROMOTIVE FORCE AND CORROSION 283

curve. The antimony-nickel curve indicates the formation of
two compounds, SbNi and SbNij, both of which retain small
further quantities of antimony, but not of nickel, in solid
solution. The lead-platinum curve, on the other hand, has
two sudden steps, corresponding with the compounds PbaPt
and PbPt, neither of which forms solid solutions. Lastly, the
lead-tin curve shows the formation of a solid solution of tin in

%4-00

Sb

Bi

sbn\

Sf, vi.j \

M

Pb

\»

Sn.

Pb^Pt

PbPt

Pt

200

I

1^ eoo

O 10 ZO 30 4-0 so €0 lO 60 90 lOO

Atom^ic ConcentraPioJV.
Fig. 93. — Typical E.M.F. curves of alloys.

lead. The form of the curve, which resembles that of a series
containing a compound, is probably due to the difficulty of
attaining equilibrium, the alloys at the lead end containing free
lead coating the crystals of the solid solution.

In the practical execution of potential measurements, the
metals and alloys are conveniently employed in the form of
rods immersed in the electrolyte contained in glass tubes out
of contact with air. The E.M.F. is measured against that of a
standard cell by the compensation method.^ A capillary

• See W. Ostwald and R. Luther, Physico-Chemkal Measurements.

284 METALLOGRAPHY

electrometer is the most convenient instrument for obtaining
compensation, especially in one of the closed forms now
available.*

The potential indicated when an alloy is first immersed
does not remain constant, owing to changes at the surface of
the electrode, but changes rapidly at first, then more slowly,
generally approaching asymptotically a constant value. It is
this value that observers have recorded, although its acceptance
as representing the true potential of the alloy is certainly open
to objection. The change is partly due to the formation of
layers of gas on the electrodes, and partly to local reactions
between the micrographic constituents of the alloy. Thus, if we
construct a circuit -of copper and platinum in a solution of
copper sulphate, the E.M.F. of the combination gradually falls
to zero, owing to the deposition of copper on the platinum.^
A metallic circuit is formed wherever two different micro-
graphic constituents are in contact at the surface of the
electrode, and such local changes must alter the potential."
The recorded data employed in the construction of the E.M.F.
curves of alloys refer entirely to such final constant values.
The alloys to be compared must be in a state of equilibrium,
and must be free from mechanical work. Under these con-
ditions, the E.M.F.-composition curve takes the following
forms : —

1. All the alloys of the series consist of conglomerates of
the pure metals. The potential is throughout that of the
more positive metal, and the curve is a horizontal straight
line.

2. The metals form a continuous series of solid solutions.
The potential varies in a continuous manner, and the curve
has a logarithmic form.

3. Solid solutions of limited concentration are formed.

' S. W. J. Smith, Phil. Mag., 1903, [vi.] 6, 398 ; H. J. S. Sand,
Trans. Faraday Soc, 1909, 5, 159.

' R. Luther, Zeitsch. physikal. Chem., 190I, 36, 385 ; F. Fischer, ibid.,
1905, 62, 55.

' See, on the formation of alloys from solutions and polarization from
this cause, F, Mylius and O. Fromm, Ber., 1894, 27, 630; A. Coehn,
Zeitsch. physikal. Chem,, 1901, 38, 609.

ELECTROMOTIVE FORCE AND CORROSION 285

The curve is smooth within the region of solid solutions, as in
(2) ; the gap in the series is represented by a horizontal portion
of the curve.

4. A single compound is formed, solid solutions being
absent. The curve is composed of two horizontal portions,
connected by a vertical line representing a sudden change of
potential.

5. Several inter-metallic compounds occur in the series,
but solid solutions are absent. The curve is composed of
several steps like those of (4).

6. Both compounds and solid solutions are formed. The
curve is a combination of (4) or (5) with (3). A vertical line,
that is a sudden fall of potential at a certain composition,
indicates the existence of a compound having exactly that
composition. If two horizontal portions are connected by
smooth curves, and not by vertical lines, the beginning and
ending of these sloping portions indicate the appearance of new
phases. If a compound forms solid solutions with both com-
ponents, there is no sudden fall of potential, and the com-
position of the compound cannot be inferred directly from the

E.M.F. OF Polymorphic Modifications

The determination of E.M.F. may also be utilized as a
means ,of detecting polymorphic change, and of measuring the
temperature of transformation. Since two polymorphic modi-
fications of a metal will, in general, differ in the amount of
energy they contain, their solution-pressures will be different
and they will assume different potentials on being brought into
the same electrolyte. This fact has been utilized in the study
of the transformation of ordinary into grey tin,^ the two modi-
fications being made the electrodes of a cell with a solution of
ammonium stannichloride, and the E.M.F. developed by the
cell determined at different temperatures.

By the use of quenched specimens, the E.M.F. of phases

• The E.M.F. of a few ternary alloys has been studied by R.
Kremann and F. Hofmeier, Monatsh., 191 1, 82, 597.

' E. Cohen and C. van Eijk, Zdtsch. physikal. Chem,, 1899, 30, 601.

286 METALLOGRAPHY

which are only stable at high temperatures, and are conse-
qently inaccessible to direct electrolytic measurements, may be
compared with that of the phases stable at ordinary tempe-
ratures. This method has proved of considerable value in
the study of the state of solution of carbon in iron under
different conditions,' and is capable of further extension.

A metal or alloy which has been subjected to rolling or
hammering, or has been in other ways mechanically strained,
has a different potential from one which is in a soft or annealed
state. iThe strained metal contains a larger quantity of energy
than the unstrained, and may therefore be expected to have
the higher solution pressure. In accordance with this, most
worked metals become the anode when connected in an
electrolyte with a piece of the same metal in an unstrained
state.^ Metals may be very severely strained by being caused
to flow through a narrow orifice. The following results were
obtained by Spring,' using such flowed metals, the comparison
electrode being a wire of the same material, heated to such a
temperature as to remove all strain : —

Sn in SnCla + o'oooii volt.

Pb„Pb(N0s)2 + O-O0OI2 „

Cd„ CdCl2 + o'ooo2o ,,

Ag „ AgNOa + 0-00098 „

Bi „ Bi(NO,)3,lHN03- 000385 „

The plus sign indicates that in the first four instances the
flowed metal became the anode. The negative sign opposite
Bi indicates that the E.M.F. developed is in the reverse
direction. Bismuth, unlike the other metals named, increases
in density when caused to flow. It is remarkable that bismuth
wire obtained in this way is sufficiently flexible to be tied in a
knot, but immediately becomes brittle on annealing.

Cadmium is so sensitive that merely rubbing the surface

' C. Benedicks, These pour le Doctoral, Ups.ila, 1904; E. Heyn and
O. Bauer, J. Iron Steel Inst., 1909, i. 109.

'^ C. E. Fawsitt, Proc. Roy. Soc. Edin., 1906, 25", 2 ; T. Andrews,
Proc. Inst. Civil Eng., 1894, 118, 356.

' W. Spring, Bull. Acad, roy. Belg., 1903, 1066.

ELECTROMOTIVE FORCE AND CORROSION 287

with emery paper is sufficient to make that specimen the anode
when compared with an untreated specimen.

Attempts to measure the E.M.F. of a metal (iron) during
the process of straining below the elastic limit have shown that
the change produced, if any, is very small, and is liable to be
masked by accidental variations.^ There appears to be a small
increase of solution pressure at the moment that elongation
takes place. The amount of energy stored up during straining
has been calculated,^ but the increase of potential theoretically
produced thereby falls within the limits of experimental
error.

A qualitative test for the presence of a given solid phase
in an alloy may be applied in some cases, by observing the
ability or inability of the alloy to precipitate another metal
from solutions of its salts.' Thus zinc, or alloys containing
the zinc phase, precipitate copper from its salts, even when
the concentration of copper ions in the latter is very small, as
in copper cyanide or cupric-ammonium salts. A solid phase
in which zinc is present in combination, however, may have
a solution pressure so low that it is unable to precipitate
copper. Thus of the alloys of zinc and copper, those con-
taining 100-59 per cent. Zn precipitate copper from all its
salts, including those in which copper is present as complex
ions. Alloys containing 55-40 per cent. Zn precipitate copper
from the ammonium compounds, but not from the cyanide or
thiocyanate; whilst those containing 38-0 per cent. Zn are
only capable of precipitating solutions in which copper ions
are abundant, such as the chloride.^ It has been attempted
to found on this basis a quantitative method of determining the
partial solution pressures of metals in solid solution, but the
process can hardly be said to have more than a qualitative
value.

' T. W. Richards and G. E. Behr, Carnegie Inst. Washi7igton, Publ.
61, 1906.

2 C. Barus, Amer. J. ScL, 1889. [iii.] 38, 193.

» O. Sackur, Arb. k. GesundheUsamt, 1904, 20, 512; 22, 187; O.
Sackur and H. Pick, Zeitsch. anorg. Chem,, igo8, 58, 46.

* 0, 3ackur, Ber., 1905, 38, 2186.

288 METALLOGRAPHY

Corrosion

With the exception of the so-called "noble" metals, all
metals and alloys are liable to be attacked and dissolved by
solutions of acids or salts. It may be considered as established
that all such corrosion is electrolytic in character, and that
the presence of substances capable of forming among them-
selves a voltaic circuit is necessary in order that solution may
take place. It is a familiar fact that metals of a high degree
of commercial purity are less readily dissolved by acids than
impure metals. Pure specimens of tin, for example, are very
little attacked by hydrochloric acid ; but the addition of a little
platinum chloride to the acid, by depositing spongy platinum
on the surface of the metal, and so setting up very numerous
local voltaic circuits, causes solution to take place very
rapidly. Quantitative experiments on the velocity of solution
of metals indicate that the process is invariably dependent on the
formation of local circuits.^ In ordinary metals, the necessary
conditions are afforded by the presence of minute specks of
impurity. A perfectly pure metal, if such could be obtained,
would probably remain unattacked by dilute acids. The
distribution of the impurity through the mass, as in a solidified
metal containing small quantities of eutectic, increases the
number of local couples and hence also the velocity of
solution.

The rusting of iron is such a process. Iron, even of the
highest attainable degree of purity, contains small particles of
carbide, phosphide, or other substance capable of forming with
the iron in presence of an electrolyte a voltaic circuit. The
presence of liquid water is necessary, as iron does not rust in
dry steam, even when the latter contains carbon dioxide.
Lastly, an acid or other electrolyte must be present, as ex-
posure to water and oxygen alone does not cause rusting.^

^ T. Ericson-Auren and W. Palmaer, Zeilsch. physikal. Chem., 1901,
39, I ; 1903, 45, 182 ; 1906, 56, 689. See also E. Brunner, ibid., 1905,
51, 95-

^ The literature dealing with the rusting of iron is extensive and con-
troversial. The experiments of G. T. Moody (Trmis. Chem. Soc, igo6,

ELECTROMOTIVE FORCE AND CORROSION 289

The electrolytic potential of a strained metal being different
from that of the same metal in an unstrained state, it is pro-
bable that the presence of strained areas on the surface of a
specimen taken for an experiment may suffice to provide the
local couples necessary for corrosion. As it is difficult to
prepare any metalKc specimen free from surface strain, this
cause may be responsible for the initiation of corrosion in
many experiments conducted with metals supposed to be pure.
It may be shown experimentally that anodic and cathodic areas
are formed on any piece of iron which is undergoing corrosion
by the following device.^ A dilute solution of potassium
ferricyanide and phenolphthalein in water is thickened slightly
with gelatin to prevent convection currents. If a piece of iron
is immersed in this reagent, blue areas appear wherever the
iron is anodic, owing to the formation of ferrous ferricyanide
where ferrous iron is passing into solution, and a pink area at
each cathode, owing to the setting free of alkali at that point.
This "ferroxyl" reagent is a sensitive means of detecting
corrosion. In an electrolyte free from oxygen, the action
soon comes to an end, owing to the formation of a layer of
gaseous hydrogen on the cathode areas, setting up an E.M.F.
of polarization, but in presence of oxygen, this layer is removed
by oxidation, and at the same time ferrous salts are eliminated
from the solution by precipitation. In water containing
carbon dioxide, the process is one of dissolution of iron as
ferrous hydrogen carbonate with liberation of hydrogen,
followed by oxidation of a part of the hydrogen and of the
ferrous salt, precipitating iron rust, which contains both ferrous

89, 720; Proc. Chem. Soc, 1903, 19, 157, 239; 1907, 23, 84) are conclu-
sive in favour of the view that iron does not rust in presence of water and
oxygen alone. See, for experiments under various conditions : W. R.
Dunstan, H. A. D. Jowett, and E. Goulding, Trans. Chem. Soc, 1905,
87, 1548; W. R. Whitney, J. Amer. Chem. Soc, 1903, 25, 394; W. H.
Walker, A. M. Cederholm, and L. N. Bent, ibid., 1907, 29, 125 1 ; J. A. N,
Friend, J. Iron Steel Inst., 1908; i. 5 ; W. H.Walker, ibid., 1909, i. 69 ;
B. Lambert and J. C. Thomson, Trans. Chem. Soc, 191 1, 97, 2426;
J. A. N. Friend and J. H. Brown, ibid., 1302.

' W. H. Walker, loc cit., and A. S. Cushman, Trans. Amer. Electro-
chem. Soc, 1907, 13, 403.

T.P.C. U

290 METALLOGRAPHY

carbonate and ferric hydroxide.^ The presence of copper,
platinum, or similar metal, facilitates corrosion, and this has an
important bearing on the means adopted- to protect iron
against corrosion. The processes of protection consist in
coating with a substance having less tendency to corrode than
iron. Leaving aside painting, which as a mechanical device
does not call for consideration here, three such substances are
in common use for the purpose, namely : zinc, tin, and the
magnetic oxide of iron, Fe304. Coating iron or steel with
zinc constitutes the process of galvanizing. Zinc alone is not
readily corroded, and galvanized iron is thereby protected.
Should any portion of the iron surface be exposed, as through
a defect in the protecting layer, a zinc-iron couple is formed.
Zinc has a much higher solution pressure than iron, and passes
into solution, the iron becoming the cathode, and therefore
remaining unattacked so long as zinc still exists in the neigh-
bourhood. Zinc plugs are even used on iron screw-propellers
and other parts exposed to corrosion by sea-water, although
it is very doubtful whether more than a small local area can
be protected by such means.

Coating iron or steel with tin is employed in the production
of ordinary tin-plate. Like zinc, tin is little corroded by mere
exposure to water containing carbon dioxide or traces of other
acids, and the iron is therefore protected so long as the tin
coating remains intact, but the behaviour of a tin-iron couple
is quite different from that of a zinc-iron couple, the iron
becoming the anode.

Magnetic oxide of iron, formed by heating iron in air or
steam, also forms a cathode, and thus hastens corrosion when
the coating has been broken through.

Alloys consisting of two solid phases in a state of mechanical
mixture obviously present the condition necessary for electro-
lytic corrosion. If we place an alloy of copper and zinc, such
as Muntz's metal, containing about 40 per cent. Zn, and com-
posed of an intimate mixture of a. and yS solid solutions, in
hydrochloric acid, local couples are at once formed, in which
the a crystals are the cathodes, and the /3 the anodes. The

' G. T. Moody, loc. cit.

ELECTROMOTIVE FORCE AND CORROSION 291

latter constituent, containing a larger proportion of zinc than
the former, is preferentially dissolved. That a surface of
Muntz's metal loses zinc and copper at approximately the same
rate ' is partly due to exfoliation of the a crystals, loosened by
solution of the inter-crystalline P constituent, and not entirely to
simultaneous solution of copper and zinc. That such simul-
taneous solution of the two metals does occur, however, when
the alloy is made the anode in an electrolyte is proved by a
series of experiments in which normal solutions of different
alkali salts were used.^ On passing a current between the

O 10 20 30 4-0 SO 60

Fig. 94. — Electrolj'tic corrosion of copper-zinc alloy

eo 30 100

Atom, % Zrt

alloy and a platinum cathode, solution of copper and zinc
occurs, causing the formation of a flocculent precipitate. The
results obtained in sodium chloride are represented graphically
in Figs. 94 and 95. In the former figure, the weight of the
corrosion product and the weight of copper in it are plotted
against the original composition of the test-pieces. The limits
of the solid solutions are indicated by vertical dotted lines.
In Fig. 95 the percentage of zinc in the corrosion product

' J. G. A. Rhodin, Trans. Faraday Soc, 1905, 1, 119.

" A. T. Lincoln, D. Klein, and P. E. Howe, J. Physical Chem., 1907,
11, 501; A. T. Lincoln and G. C. Bartells, ibid., 1908, 12, 550; TrKus.
Amer. Electrochem. Soc, 1908, 13, 331 ; A. T. Lincoln, ibid., 11, 43.

292

METALLOGRAPHY

is plotted against the percentage in the original specimen. If
the alloy contains less than 40 atomic per cent. Zn, the corro-
sion product has the same composition as the test-piece, which
is therefore unaltered by corrosion. These alloys correspond
with the a and a + fi series of solid solutions. With the
appearance of the y phase, the percentage of copper removed
drops almost to zero, zinc only being dissolved, and a layer
rich in copper being formed on the surface of the anode.
This layer may scale off as corrosion proceeds, so that if the
corrosion were determined by the loss of the anode, instead of

ON

~^

1 1

s 1

l'>^

1 N. 1 . '

^

1 'f"- ' 1 '

, V 1 1 1

1

1 \, ' 1

** t 1

j 1

1 1 '^>

ex

.1.1

^^

~i 1 L^H

O 10 ZO 30 *0 SO 60 70 60 SO lOO
Atom.ic%Z7\. in- alloy.

Fig. 95. — Electrolytic corrosion of copper-zinc alloys.

by analysing the corrosion product, it would appear that copper
as well as zinc was removed from the anode. A few tests of
the chemical corrosion were made by the same authors, no
current being passed, but the results obtained were indecisive.
Microscopical examination, however, proves that the surface
is by no means equally attacked. Alloys consisting of the
a + /3 phases, within which range Muntz's metal and similar
alloys fall, are at first attacked only on the /3 areas, the zinc
from which is rapidly removed, leaving an alloy very rich in
copper, and not until this change has penetrated to a con-
siderable depth is the a constituent attacked, undergoing a

ELECTROMOTIVE FORCE AND CORROSION 293

similar change. The nature of the process demands further
investigation, but it is not justifiable to assume that chemical
corrosion, due only to the influence of local couples, follows
exactly the same course as corrosion under the influence of a
considerable external E.M.F.

The addition of tin to copper-zinc alloys is frequently
adopted with the object of lessening the corrosion in such
cases as that of bolts exposed to the action of sea-water, the
quantity of tin amounting to i to i"5 per cent. In Lincoln's
experiments the tin was found to exert very little influence on
the corrosion, but it is improbable that this also applies to
simple chemical corrosion. The effect of the addition of tin
up to I "4 per cent, to an a, -1- (3 copper-zinc alloy is to displace
the limits of the a and /3 phases, without altering the structure
in any other way, the tin being about equally distributed
between the two phases, whilst the addition of any larger
quantity of tin leads to the appearance of a new constituent,
an alloy of copper and tin having approximately the composi-
tion CujSn.*

The electrolytic corrosion of the alloys of copper and tin "
also shows discontinuities corresponding with the disappear-
ance of certain solid phases. The alloys rich in tin frequently
become passive owing to the formation of an insoluble layer
of stannic oxide. Considerable exfoliation takes place from
the surface of some of the copper-tin, as well as of the copper-
zinc alloys, and in further investigations of this kind it will be
necessary to distinguish clearly between the removal of metal
by true solution and by mere mechanical flaking.

' L. Guillet, Rev. de MHallurgie, 1906, 3, 243.

= B. E. Curry, J. Physical Chem., 1906, 10, 474 ; Trans. Amer.
Electrockem. Soc, 1906, 9, 173 ; F. Giolitti and O. Ceccarelli, Gazzetta,
1909, 39, ii., 557. The corrosion of aluminium-copper alloys has been
examined in a similar manner by W. S. Rowland, J. Physical Chem., 1908,
12, 180.

294 METALLOGRAPHY

The Chemical Examination of Residues

The action of a chemical reagent on an alloy may be
continued until the reagent ceases to dissolve anything further,
and the nature of the residue remaining may be investigated
as a means of determining the proximate composition of the
original alloy. For this purpose the material is generally
reduced to a fine state of division by filing or powdering, and
is treated with successive quantities of the reagent so long as
any metal passes into solution. This method was formerly a
favourite one for determining the formute of the inter-metallic
compounds contained in alloys. For example, a series of
alloys of a metal with zinc of varying composition, but pre-
sumed to contain an excess of free zinc, would be extracted
with dilute hydrochloric acid to remove this excess, and the
residues submitted to analysis. If the residues from alloys
originally differing from one another proved to have the same
composition, a formula was calculated and assigned to the
substance unattacked by the reagent. The greater number of
the formulae of inter-metallic compounds found in text-books
of inorganic chemistry have been arrived at by this means,
and the method is still frequently applied by French chemists,
and other isolated examples of its use occur from time to
time.' There are, however, numerous objections, of a theo-
retical and practical character, to the employment of such a
method, and it can at most be admitted as an auxiliary in
certain cases. Formulae based solely on the behaviour of
alloys towards reagents, must therefore be refused recognition
until confirmed by more trustworthy methods.

To deal with the practical difficulties first, it is frequently
the case that the action of the reagent is brought to a stand-
still after . a time by purely mechanical hindrances. For
example, the extraction of free silicon from alloys by means
of alkali hydroxide or carbonate has been repeatedly employed
to fix the formula of metallic silicides. It has been shown,

•
' For example, in a study of the silicides of copper, M. Philips,
MetaViirgk, 1907, 4, 587, 613.

ELECTROMOTIVE FORCE AND CORROSION 295

however, by Guertler ^ that the action of the alkaUne solution
is not confined to the free- siHcon, but that the silicides are
also slowly attacked. The removal of Combined silicon leads
to the formation of a layer of metal on the particles of the
residue, preventing further action. The analysis of the
residue after such treatment obviously does not correspond
with the formula of the silicide. By treating the residue with
dilute acid to remove the metallic layer, and repeating the
extraction with alkali, the whole of the alloy may generally be
brought into solution. Similar objections apply to the extrac-
tion of alloys with acids, bromine water, etc.

Apart from these practical difBculties, which are sufficiently
serious, the method is defective in principle. The preceding
sections of this chapter have shown how complex is the action
of reagents, whether alone or assisted by an electric current, on
alloys. The relation of the composition of the dissolved and
undissolved portions depends on a number of factors, and
there is no reason whatever to suppose that the residue after
attack consists of a pure inter-metallic compound. Even if
proved to be homogeneous by physical tests, it may be a solid
solution in equilibrium with the reagent under the given
conditions. Rather more weight may be attached to the
results if the attack by several different reagents — acids and
alkalis, for example — yields residues of identical composition,
but this is rarely the case.

An inter-metallic compound is not necessarily less acted
on by reagents than the more reactive of its component metals,
although this is true of many compounds. Guertler enumerates
the following compounds which are more readily tarnished by
moist air, or acted on by dilute acids,' than either of their
components: PbTlj, Bi/flg, NaPb, NagBi, Na^Sb, Mg^Pb,
MgaSn, Mg3Tl2, CusSi, LiaSe, several compounds of calcium,
and the compound of lead and platinum richest in lead, the
formula of which is unknown.

Still less trustworthy is the method of pouring off the
liquid portion of a partly solidified alloy, and treating the
crystals with some reagent to remove adhering solidified

' Metallurgie, 1908, 5, 184, 621 ; compaie E. Rudolfi, ibid., 257.

296 METALLOGRAPHY

mother-liquor. A clean, glistening appearance of the solid
thus obtained has often been accepted as proof of its chemical
individuality, especial]^ if the crystals exhibit some degree of
stability towards reagents. Chemical literature is very rich in
compounds described on such evidence alone. It is clear
from a consideration of the equilibrium in alloys that a well-
crystallized solid phase is by no means necessarily a chemical
compound.

Heat of Solution

The formation of alloys from their components may take
place either with development or absorption of heat. The
former case is observed in the combination of sodium with
mercury to form the compound NaHga, and the latter in the
preparation of many fusible metals and amalgams. For
example, Dobereiner observed in 1824,^ that on mixing two
amalgams of lead and bismuth at 16°, the temperature fell to
— 6"5°, whilst by mixing tin, lead, bismuth, and mercury a fall
of 23° could be obtained, and he correctly pointed to the
analogy with freezing-mixtures. Very few calorimetric estima-
tions of the heat developed or absorbed have been made,
although a few such are due to Person.^ The direct estimation
is only practicable in the case of metals which alloy at a low
temperature, the calorimetric difficulties in other cases being
almost insuperable. The possibility of other reactions con-
tributing to the heat change must not be overlooked. Thus,
when aluminium is added to molten copper, there is an
immediate rise of temperature, often sufficing to raise the
whole mass to an intense white heat, but this must not be
attributed to the heat of combination of copper and aluminium,
which is probably quite small, but to the reduction of the
copper oxide, which is always present, by the aluminium, a
strongly exothermic reaction. It is indeed very difficult to
judge, by direct experiments alone, whether the heat change

' Schweigg. y., 1824, 42, 182.

' Anti. Physik., 1849, [ii ] 76, 586.

ELECTROMOTIVE FORCE AND CORROSION 297

accompanying the formation of an alloy is positive or
negative, and the information must therefore be sought in-
directly.

The indirect method of determining the heat of combina-
tion consists in measuring the heat developed when each alloy
is dissolved in an appropriate reagent, and comparing the
result with that calculated for a mixture of the same com-
position from the observed heats of solution of the component
metals in the same solution. If the observed heat of solution
is less than that calculated by the rule of mixtures, the difference
is assumed to represent the heat developed in the formation
of the alloy. This method, originally due to Hess,^ has
been employed by several investigators. The alloys of zinc
and copper, in particular, have been examined repeatedly in
this way. Dilute nitric acid was at first used as the solvent,^
but it was shown ' that the gaseous nitrous products formed
vary with the composition of the alloy, so that the reactions
throughout the series are not comparable, and consequently
no deduction can be made as to the heat of combination. A
more extensive series of measurements, including alloys of
other metals, was made by Herschkowitsch,'' who used a
solution of bromine in potassium bromide as the solvent.
The number of points on each of his curves is, however, too
small to justify any conclusions. Very careful calorimetric
measurements, using ferric ammonium chloride and cupric
ammonium chloride as the solvents," indicate a maximum
development of heat in the copper-zinc alloys at a composition
corresponding with the formula CuZnj, the molecular heat of
formation of which is found to be 10,143 cal.

The heat of formation of the carbides and silicides of iron

' Ses 0%iv;3\i.'s Xlassiker der exacttn XVissenschaften, No. 9, 1890.

2 A. Gait, Bnt. Assoc. Rep., 1898, 246 ; 1899, 787 ; Proc. Roy. Soc.
Edin., 1898,23, 137.

' J. H. Gladstone, Phil. Mag., 1900, [v.] 60, 231.

< Zeitsch. physikal. Chem., 1898, 27, 123.

* T. J. Baker, Phil. Trans., 1901, 196a, 529 ; see also W. F. Luginin
and A. Schiikareff, Arch. Sci. phys. nat., 1902, [iv.] 13, 5 ; 1903, [iv.]
15, 49.

2g8 METALLOGRAPHY

has been studied in the same way, by dissolving in cupric
ammonium or potassium chloride.'

Further experiments on these lines may lead to results of
interest, but it is necessary that the calorimetric method
adopted shall be of high accuracy, and that the reagent used
shall be one of which the action is well known and uniform
throughout the whole series studied.

' E. D. Campbell, J. Iron Sleel Inst., igoi, i. 211.

CHAPTER XIV

THE CONSTRUCTION OF THE EQUILIBRIUM DIAGRAM

The complete equilibrium diagram of a series of alloys, having
temperature and concentration as its co-ordinates, is composed
of a number of lines and areas, the position of each of which is
fixed by appropriate experimental means. The lines com-
posing the diagram and bounding the areas of phase stability
are, the vapour phase as usual being neglected —

1. The freezing-point curve, or liquidus;

2. The solidus, which is also regarded, somewhat loosely, as

the melting-point curve ;

3. Curves of liquid solubility, separating the regions of

immiscible liquid phases ;

4. Horizontal or inclined lines, representing the transform-

ation of solid phases, such as the polymorphic changes
of crystals, the formation of compounds from solid
constituents, and the separation of new phases from
solid solution ;

5. Vertical lines, representing the limits of concentration

between which the solid phases occurring in the system
are stable in contact with each other. When the
phases in question are not pure metals or compounds,
but solid solutions, these dividing lines are slightly
inclined, since the limits of saturation vary to a
slight extent with the temperature. The variation is
frequently negligible, and the lines may be regarded
as vertical. When the variation is appreciable, the
bounding lines are to be regarded as falling under
class 4.

299

30O ; METALLOGRAPHY

In the simplest cases, the positions of all the lines
enumerated above may be fixed by a thermal and micro-
scopical study of the alloys alone, although the possibility of
changes which these methods fail to detect must not be over-
looked. In more complex cases, ambiguities may arise in the
interpretation of thermal and microscopical results unless the
variations of other physical properties, such as specific volume,
hardness, electrical conductivity, magnetic susceptibility and
electromotive force are utilized to furnish auxiliary data. The
results of such measurements may only serve to confirm con-
clusions already arrived at ; but in other instances new light is
thrown on the constitution of the alloys, and a metallographic
investigation of any series of alloys cannot be regarded as
complete unless its scope embraces the more important
physical properties. Moreover, accurate data are urgently
needed at the present time as a basis for generalizations as to
the relations between the constitution of alloys and their
physical properties. The establishment of such relations
would have a high technical value, in making it possible to
predict the character of new alloys and to determine the com-
position of the alloy most likely to present a certain required
combination of properties. Furthermore, the question is one
of fundamental importance for inorganic and physical chemistry.
There are many theoretical problems on which light would be
thrown by a thorough and systematic investigation of the
properties of metallic alloys. The nature of the processes of
solution, and of the attraction between solute and solvent, is
apt to receive one-sided treatment when liquid solutions alone
are considered, whilst no theory of valency or of chemical
affinity can be admitted as satisfactory which does not take
account of the remarkable relations exhibited by the inter-
metallic compounds. The polymorphic changes undergone by
solidified alloys afford a critical test of theories of crystalline
structure and of the marshalling and rearrangement of closely
packed assemblages, and attempts to explain the properties of
solids by the application to them of a modified van der Waal's
equation or of theories of internal pressure, must stand or fall
by their applicability to alloys. The investigation of metals

CONSTRUCTION OF EQUILIBRIUM DIAGRAM 301

and alloys subjected to mechanical strain has brought new
elements unto the long-standing controversy as to the meaning
of " solid " and " crystalline " states of matter, a controversy
which received new life from the discovery of liquid crystals.
Again, the nature of the molecular arrangements which give to
magnetic bodies their remarkable properties must be considered
in the light of the fact that non-magnetic metals are now known
to form strongly magnetic combinations. Lastly, the nature
of metastable and labile conditions and of false equilibria, and
of their dependence on viscosity, is advantageously studied
in metallic mixtures. The geologist, hampered by the high
temperatures and enormous viscosities which present themselves
in the experimental study of rocks, turns to alloys for information
as to the processes which take place in the cooling of igneous
magmas, and the changes in structure which are brought about
by a departure from a condition of equilibrium. These instances
of theoretical problems, in the solution of which the metallo-
graphist can render valuable aid, might be supplemented by
many others.

The diagram considered has certain inevitable limitations.
It is essentially an equilibrium diagram, that is, it represents the
limits of temperature and composition within which each phase
is, in the strict sense of the word, stable. But stability pre-
supposes equilibrium conditions. An alloy may prove on
examination to differ widely in properties from an ideal alloy
having the same composition and the same temperature, but
its condition is then not one of stability. The changes
necessary to reach a state of stability may proceed so slowly,
on account of tbe low velocity of diffusion, that the alloy may
present a false appearance of being in equilibrium. Experi-
ments continued over a long period may be required to
determine the true equilibrium. Thus the alloys of lead and
tin, frequently cited in the older text-books as a typical series
composed of mutually insoluble solid metals, were found by
later investigators to contain solid solutions of limited concen-
tration ; but baking for six weeks at a temperature only 2° below
the eutectic point is necessary to enable the solid solutions,
even in slowly cooled alloys, to attain their equilibrium

302 METALLOGRAPHY

concentration.' The complete diagram having once been con-
structed, however, an inspection of it affords at least qualitative
information as to the metastable and labile conditions which
may be obtained, for instance, by rapid cooling. The prolong-
ation of branches of the liquidus curve below the eutectic
point indicates the effect of undercooling, and points to the
abnormalities which may be expected in the micro-structure.
The suppression of a transformation by rapid cooling through
a certain range of temperature, causing the retention in the
cooled alloys of a phase only stable at a higher temperature, is
aimed at in all " quenching " processes, and a comparison of the
microscopic structure with the diagram will indicate the extent
to which the transformation has been suppressed. Such
systematic quenchings, if sufficiently rapid, may be employed
in the construction of the equilibrium diagram itself, as will be
shown later.

After these general remarks, we may take the component
curves and bounding lines of the diagram in order, enumerating
in each instance the means which are at the disposal of the
metallographist for fixing their positions.

I. The Freezing-point Cnrve, or lAquidus. — The construction
of this curve is always based on the determination of the cooling
curves of the individual alloys. " Direct" cooling curves (p. 123),
preferably brought to a regular form by Plato's device, or the
" inverse-rate " curves derived from them, are the most suitable
for the purpose, as it is the initial freezing-point which is required.
If undercooling takes place, the development of heat when
freezing begins may be insufficient to raise the temperature of
the mass to the true freezing-point, it is therefore necessary to
guard against undercooling by inoculation with the solid phase.
For this purpose, a preliminary cooling curve is taken, and the
approximate freezing-point determined. A small portion of the
solid alloy is reserved and reduced to powder, the remainder of
the mass is then re-melted and the cooling curve taken. As the
freezing-point is approached, particles of the solid are introduced,
the mass being thoroughly stirred.

The thermo-couple used should be calibrated by taking the

' W. Rosenhain and P. A. Tucker, Phil. Trans., 1908, 309a, §9.

CONSTRUCTION OF EQUILIBRIUM DIAGRAM 303

freezing-points of a number of pure metals, using the same
apparatus as in the investigation. Small errors due to a lag of
temperature between the thermo-couple and the alloy are
eliminated in this way; large errors should not occur if the
tube protecting the couple is of small diameter and the rate of
cooling is sufficiently low. This calibration by means of pure
metals, using the standard freezing-points given on p. 114,
affords a ready means of reducing all readings of temperature
to those of the air thermometer. The reduction should always
be performed, the publication of thermal diagrams referred to
more or less arbitrary standard temperatures, a too frequent
practice, being very misleading.

It is of great interest to determine, with a high degree of
accuracy, the initial portions of a freezing-point curve, as was
done by Heycock and Neville for a large number of metals, the
pyrometer used by them being of the very sensitive platinum re-
sistance type. By taking a sufficient number of points at a short
distance apart, the atomic depression may be determined, and
the validity of Raoult's law tested. Information is thus gained
as to the molecular condition of the dissolved metal (see p. 329)
Similar closely-grouped determinations in the neighbourhood
of maxima in the liquidus also present a certain interest as a
means of estimating the degree of dissociation of an inter-
metallic compound into its components on melting. Alloys
have not yet been studied from this point of view, but the
researches of Kremann on mixtures of organic substances
indicate that interesting results may be expected, especially if
a comparison be made with ternary systems.

Should the freezing-point curve be horizontal for a part of
its course, the existence of two liquid phases may be suspected,
and one should proceed as under section 3 below.

The most difficult part of a freezing-point curve to deter-
mine is a branch ascending very rapidly from a eutectic point.
Such branches are most likely to occur in systems in which the
eutectic composition lies very close to one of the component
metals. This is the case, for instance, in the alloys of copper
and bismuth, the eutectic mixture of which is practically indis-
tinguishable from pure bismuth. The curve representing the

304 METALLOGRAPHY

crystallization of pure copper rises very rapidly at first — to the
extent of 400° for 20 atomic per cent. Cu.^ The initial freez-
ing-point of an alloy falling within this range is marked by
only a very indistinct arrest on the cooling curve, since the
quantity of copper deposited is very small. The use of large
quantities of the alloy, a slow rate of cooling, a sensitive gal-
vanometer, and the plotting of the results in the form of
" inverse-rate " curves, are to be recommended in such cases.

The freezing-point surface of a ternary system is constructed
by grouping the results of the thermal study so as to form a
series of binary systems, or vertical sections through the space-
model (p. 76).

It is often recommended that the determination of the
cooling curve should be supplemented by that of the heating
curve. Whilst this undoubtedly gives valuable results in the
study of polymorphic change, it is of far less value for the con-
struction of the liquidus. The initial freezing-point on cooling
corresponds with the disappearance of the last solid particles
on heating, a point which it is difficult to determine with the
requisite accuracy.

The liquidus represents the limit of existence of solid
phases. All points lying above it denote completely liquid
alloys, assuming that the temperature is not raised above the
boiling-point of one of the components. Each point on the
liquidus gives the highest temperature at which a solid phase
can exist in an alloy of that composition. All points below
the liquidus denote alloys in which, under conditions of equi-
librium, at least one solid phase is present.

It is sometimes impossible to complete the liquidus of a
series of alloys, on account of the volatility of one of the com-
ponents at temperatures approaching the melting-point of the
other component. An investigation of the alloys of iron and
zinc, for instance, is necessarily limited, under ordinary condi-
tions, to the alloys ranging from pure zinc to a mixture con-
taining a small percentage of iron ; richer alloys lose zinc before
melting. The alloys of arsenic with the less fusible metals, on
the other hand, can only be investigated at the end of the
' K. Jeriomin, Z^Vj'f.i, nnorg. Chem,, 1907, 55, 412.

CONSTRUCTION OF EQUILIBRIUM DIAGRAM 305

series corresponding with the higher temperatures, the alloys
containing small proportions of arsenic being stable, in spite of
their high melting-point, whilst those rich in arsenic lose the
excess of this element when melted under atmospheric pres-
sure. It is possible, when the general aspect of the diagram is
known, to construct the portion inaccessible to direct experi-
ment by extrapolation from a ternary diagram. This device,
which has been applied in a single instance, may prove of
value in other researches into similar systems. The alloys of
zinc and nickel can be examined under ordinary pressure up to
54 per cent, of nickel, beyond which the melting-point is so
high that zinc is lost by volatilization. The ternary system
copper-zinc-nickel has been investigated, and the increase of
fusibility due to the copper makes it possible to determine the
freezing-points of almost the whole series of ternary alloys.^
The general course of the liquidus and soHdus has been deter-
mined as far as the volatility of the zinc allows, and extrapola-
tion of the surfaces obtained gives the missing portion of the
diagram of the binary system nickel-zinc without serious risk
of error.

2. The Solidus. — The construction of the solidus, dividing
those regions in which only solid phases are present from those
in which liquid is still present, presents much greater difficul-
ties, both experimental and of interpretation, than that of the
liquidus. The solidus is made up, in the majority of cases, of
a number of separate portions — horizontal, vertical, and in-
clined. Taking the simplest case first*that of a eutectiferous
series in which the only solid phases are the pure components
(Fig. 4), the only line which falls to be determined is the
eutectic horizontal. The points on this line are given by the
lower arrests on the cooling curves. With sufficiently accurate
experimental methods, all of these arrests should occur at the
same temperature, although small deviations are liable to occur
as the ends of the horizontal are approached, that is, as the
quantity of eutectic diminishes. A steady fall of the eutectic
line in one direction points to experimental inaccuracy.'^ The

' V. E. Tafel, Metallurgie, 1908, 5, 343, 375, 413.
'^ See, for instance, P. Dejean, Rev. de Mitallurgie, 1906, 3, 233,
T.P.C. 5i

3o6 METALLOGRAPHY

determinations nearest to the eutectic point, where the arrest is
most strongly marked, are likely to be the most accurate, and
should be chiefly trusted in fixing the position of the horizontal.

Undercooling at the eutectic point is a source of consider-
able difficulty. A pure eutectic mixture, like a pure metal,
may be undercooled, and the addition of both solid phases
simultaneously is necessary to prevent the undercooling. This
is effected by the device of adding a portion of the pre-
viously cooled and powdered alloy; but when the eutectic
forms only a small part of the alloy, this precaution is difficult
to apply. When one phase is in excess, it may continue to
separate after the eutectic temperature has been passed, the
remaining constituent continuing as an undercooled liquid.
The final solidification of the latter is marked by a rise of
temperature, by which the fact of the undercooling is rendered
visible on the cooling curve, although its extent remains
doubtful. The difficulty of resorting to inoculation is due to
the fact that the liquid eutectic is distributed throughout a
mass of the solid excess constituent, and is not readily brought
into contact with the substance used for inoculation, although
this may be partly remedied by thorough stirring. The
existence of eutectic undercooling may sometimes be detected
by the two branches of the freezing-point curve appearing to
intersect at a point somewhat above the horizontal drawn in
the preliminary diagram.

In cases of this kind a useful check is afforded by the
examination of heating curves. When the eutectic tempera-
ture is reached in heating, a portion of the alloy melts ; and
this takes place without superheating, provided that segregation
has not been so excessive as to prevent intimate contact of the
solid phases. If segregation has taken place, the apparent
eutectic point is too high (p. 29). The correct temperature
may be obtained by taking a heating curve after reducing the
alloy to a fine state of division.

The temperature of the eutectic horizontal having been
fixed, it remains to determine the limits of composition to

where the rise of the copper-copper oxide eutectic line is certainly due to
jin error, probably occasioned by considerable undercooling.

CONSTRUCTION OF EQUILIBRIUM DIAGRAM 307

which it extends in each direction. The component metals
are generally capable of retaining a certain quantity of the
other component in a state of solid solution, and in such a
case the eutectic line must stop short of the limits of the dia-
gram. Microscopical examination is a more sensitive method
of fixing these limits than thermal analysis. Should a slowly
cooled alloy of A and B, containing, for example, o"i per cent,
of the metal B, show under the microscope distinct traces of
inter-crystalline matter, we may conclude that the solubility of
B in solid A does not exceed o'l per cent. The application
of this method has often failed from the difficulty experienced
in many cases of preparing a sufficiently good surface for so
minute a quantity of a micrographic constituent to become
visible. This is purely an experimental difficulty, and is cer-
tainly not insurmountable. With the necessary experimental
skill, and by the use of suitable polishing methods, especially
when dealing with soft metals such as lead, it is undoubtedly
possible to detect very minute traces of included inter-crystal-
line matter. Heat-tinting or exposure to vapour is to be pre-
ferred for this purpose to etching with acids, since the latter
treatment results in the production of grooves between the
crystal grains, masking the feature sought for. The included
metal B is more likely to appear as a film or band than to
show a eutectic structure, on account of the tendency of eutec-
tics, already noted, to undergo segregation in presence of a
large excess of one of their component phases.

The second method of fixing the limits of the eutectic hori-
zontal is that due to Tammann, consisting in plotting the dura-
tion of the eutectic arrest in each alloy of the series against the
composition. It has the advantage that should the arrest
become indistinguishable in the extreme members of the
series, the limits of the horizontal may be estimated by extra-
polation. The trustworthiness of this method has been keenly
debated,^ but an examination of the results obtained by it leads
on the whole to the conclusion that the method is unexception-
able if applied with proper experimental precautions, the chief

' See a discussion on papers by W. Rosenhain and by C. II. Desch, in
J. Inst. Metals, 1909, i. Z40.

3o8 METALLOGRAPHY

requisites being a pyrometric method of high sensitiveness and
the employment of comparatively large quantities of material.
The excellent results obtained in the laboratory of Friedrich,
of Freiburg, by the application of this method, are strongly
in its favour, and the criticisms which have been raised against
it are due rather to hurried publications by insufficiently
skilled observers, based on work with small quantities of
impure material, than to defects inherent in the method itself.

In applying the method of eutectic times, it is necessary
to ensure that all the alloys of the eutectiferous series are
examined under comparable conditions. The quantities of
component metals should be so chosen that the molten alloys
occupy equal volumes, it being advisable to determine this by
a preliminary experiment. If very different volumes of the
successive alloys of a series are taken, the correction of the
arrest-times by calculation for equal volumes is unsatisfactory,
on account of the uncertainty introduced
into the cooling conditions.

The cooling curve usually presents a
slight rounding of the horizontal portions,
and it is therefore necessary to determine
what are the limits of time between
which the arrest is to be assumed to
continue. This may be done with a satis-
factory degree of accuracy by producing
the horizontal line be (Fig. 96) and the
descending line de until they meet at g.
Fro. 96. i^iie distance bg. measured on the time

Correction of coolinsr , , , , , . ,.

curve. scale, may be taken as the duration of

the arrest.
When the arrest-times are plotted against their respective
compositions, the curve joining the points thus obtained
should consist of two straight lines, meeting at the composition
of the eutectic mixture, and intersecting the axis of time at
the compositions at which the last trace of eutectic disappears.
Such a condition is shown in Figs. 4 and 7. If the eutectic
composition practically coincides with one of the pure metals,
the curve consists of only one such line. Sometimes the line,

CONSTRUCTION OF EQUILIBRIUM DIAGRAM 309

or lines, instead of being straight, are markedly curved, the con-
cavity being usually directed downwards, towards the composi-
tion axis. Tammann suggests that the eutectic limits may still
be found in such a case by extrapolation, but the uncertainty of
such a procedure is obvious, since the curve tends to flatten
as it approaches the axis, and little weight can be attached to
limits calculated in this fashion. The deviations from a
straight line are probably due to variations in the specific heat
from one alloy to another, and they should therefore disappear
wholly or largely from curves taken by Plato's method.
Limits found by extrapolation should only be employed in
the construction of an equilibrium diagram when the lines
produced are sensibly straight. A detailed example of the
application of the method is given below (p. 320).

In addition to eutectic lines, the solidus may comprise
lines corresponding with chemical reactions between solid and
liquid, such as the line at 460° in the diagram of the antimony-
gold alloys (Fig. 13, p. 40), which indicates the chemical
reaction

AuSba^Sb -I- liquid.

Such a reaction, occurring during cooling, takes place in the
direction of the lower arrow with development of heat, and is
consequently marked by an arrest on the cooling curve, an
arrest which is very liable to be affected by under-cooling.
The treatment of the arrest times follows that described above
in connection with eutectics. The crystallization time is a
maximum at the formula corresponding with the compound
formed on cooling. The reaction is liable to be incomplete,
owing to the fact that one of the reacting bodies is solid, the
product being also solid. A reaction therefore takes place on
the outer surface of the crystals, a layer of the new compound
being formed, but the presence of this layer checks further
chemical action, by isolating the interior of the crystals from
the liquid with which they ha^e to react. Unless sufficient
time is allowed for the composition of the particles to become
uniform through diffiision, the process is arrested at an incom-
plete stage. When this is the case, the maximum development

310 METALLOGRAPHY

of heat due to the reaction is not observed at the composition
of the compound, but at the composition of an alloy contain-
ing a larger quantity of the more fusible metal, as the thermal
effect is the same as if a part of the original crystals had been
withdrawn from the, mass. Evidently, the error due to this
cause will be lessened by thorough stirring, which favours the
formation of very small crystals, presenting a larger surface in
proportion to their mass, and therefore allowing the reaction
to proceed further towards completion. The plan of causing
cooling to proceed so slowly that diffusion has time to equalize
the composition of the solid particles is impracticable on
account of the slowness of the process. We may also, after
cooling the alloy, crush it to a fine powder to facilitate contact
of the reacting substances, and re-heat to the transformation
temperature, taking a new cooling curve from that point.*
Eutectic arrests occurring at lower temperatures, which are
displaced as to duration by the error described, assume their
correct values after this treatment.

Turning to alloys in which solid solutions are formed,
the determination of the exact position of the solidus now
assumes greater difficulty. Consider first a pair of isomorphous
metals, such as gold and silver. The solidus may be con-
sidered from two points of view. On the one hand, it repre-
sents the temperatures at which solidification is completed ; on
the other, it represents the composition of the solid phase in
equilibrium with the liquid at each temperature. An experi-
mental determination by the latter method is impracticable, for
although means have been devised for isolating and analysing
solid phases during crystallization,^ it is impossible to free the
crystals from mother-liquor, and the accuracy obtainable is
quite insufficient for the purpose. The thermal method is
therefore the only one available. Its uncertainty arises from

' G. Tammann, Zeitsch. anorg. Chem., 1905, 45, 24. An instance
occurs in the alloys of gold and lead at the point of formation of AujPb.
R. Vogel, ibid., 1905, 45, II. '

'^ C. van Eyk, Proc. k. Akad. Wetensch. Amsterdam, I902, 10, 859. A
device for estimating the proportion of mother-liquor retainediis described by
A. vanBiljert, Zeitsch. physikal. Chem., 1891, 8, 343 ; andW. D. Bancroft,
^. Physical Chem., 1902, 6, 178.

CONSTRUCTION OF EQUILIBRIUM DIAGRAM 311

the fact that the point c on the cooling curve (Fig. 17), at
which the development of heat ceases and the curve joins
that of the cooling solid, is marked by only a slight change of
curvature, so that its position is not readily determined. The
more sensitive the instruments used, and the more nearly the
directions of ab and cd approach to a straight line, the more
distinct becomes the change of curvature. The position of the
solidus in all such diagrams, however, should be accepted with
considerable reserve. A further error, due to imperfect read-
justment of equilibrium between crystals and liquid during
solidification, has already been described.

An ingenious method of fixing the position of the solidus,
different in principle from those described, was introduced by
Heycock and Neville in their investigation of the copper-tin
alloys.^ It consists in quenching several alloys of the same
composition from different, exactly determined temperatures,
lying a short distance below the initial freezing-point. One of
these alloys, let us suppose, is quenched before it is completely
solid, and on microscopical examination it is easy to detect the
suddenly chilled mother-liquor as an inter-crystalline material,
filling the cavities between the crystals which separated during
the slow cooling. Another alloy, quenched just after solidifica-
tion was complete, is found to consist wholly of the solid
solution, The point on the solidus corresponding with this
alloy evidently lies between the quenching temperatures of
these two alloys, and by a suitable choice of temperatures the
solidus may be drawn with great accuracy if adequate means
of rapid quenching are available and a sufficient number of
alloys are examined. The method has been little followed by
later workers, although it is unassailable. It has recently been
used with success in fixing the position of the solidus in the
iron-carbon series, thermal methods having failed to give a
sufficiently exact result.^

It has been said that the solidus may also be regarded as

' Phil. Trans., 1903, 302a, i.

* N. Gutowsky, Metallurgie, 1909, 6, 731, 737. . The same method has
been employed in a very careful study of the ternary alloys of copper,
tin, and aluminium, C. A. Edwards and J. H. Andrew, J. Inst. Metals,
1910, 2, 29.

3 12 MEtAtlOGRAPHV

the melting-point curve, since it represents the temperatures at
which the first appearance of a liquid phase takes place when
the alloys of the series are heated. The exact determination
of the first appearance of liquid is a matter of difficulty. The
thermo-couple, being in contact only with the solid alloy, does
not respond readily to the small change in the rate of heating
which marks the melting of the most fusible constituent, and
the equalization of temperature throughout the mass by stir-
ring is impossible. As a rough preliminary determination of
the solidus, especially in the case of alloys fusing at a moderate
temperature, the method of heating the alloy in a furnace of
uniform temperature may be adopted, the first appearance of
"sweating" on the surface of the alloy being noted. The
temperature thus observed is above the true value, but a series
of such observations may be of value in indicating the existence
of a compound in the series, an abrupt change in the tempe-
rature at which liquid appears occurring when the composition
of the compound is passed. Such a curve has been used with
success in the investigation of mixtures of non-metals, under
the name of the " sintering-point curve." ^

The dilatometric method may also be employed to fix the
position of the solidus in the case of alloys which can be
heated in a volume-dilatometer. This method is therefore
most suitable for alloys of low melting-point. It has been
applied to the cadmium amalgams,^ the volume changes of
which on melting are considerable.

3. Cwves of Liquid Solubility. — The separation of an alloy
into two liquid phases, or the failure of two metals when
melted together to form a homogeneous liquid, introduces an
important modification into the thermal and microscopical
results. Should such a separation be suspected, the mixture
is allowed sufficient time for the separation into two layers to
take place, and is then allowed to cool and examined micro-
scopically. The difference between the upper and lower
parts of the ingot, supposing separation to have taken place,
is usually obvious. It may happen, however, that the two

' A. Stock, Ber., 1909, 42, 2059, 2062.

' H. C. Eijl, Zeitsch. fhysikal. Chem., 1902, 41, 641.

CONSTRUCTION OF EQUILIBRIUM DIAGRAM 313

liquids form an emulsion which separates with difficulty, the
less fusible alloy then solidifies as a spongy or honeycomb-like
mass, in the meshes of which the still liquid constituent is
held. This may be detected microscopically, or even by visual
examination without magnification. The drops or globules of
the more fusible constituent are not liable to be mistaken for
the crystals which separate from a homogeneous liquid. The
mixture may be very intimate, as in some alloys of copper,
tin, and lead used as bearing-metals,^ but their character of
emulsions is unmistakable.

If any portion of the freezing-point curve has been found
to have a horizontal direction, the formation of two liquid
layers may be suspected. A short horizontal branch may also
exist in the case of a compound completely dissociated at its
melting-point under exceptional conditions,^ but the case is
unlikely to present itself in alloys. Mixtures having a com-
position falling between the limits of the horizontal portion are
therefore submitted to microscopical examination.

The maximum duration of the corresponding arrest occurs
at the point E (Fig. 37), the arrest being due to the crystalli-
zation of the metal B from the liquid phase E. The quantity
of this phase which is produced is a maximum at E. The
development of heat due to separation into two liquid phases
is in general very small, and the cooling of an alloy will there-
fore not undergo a marked arrest on crossing the curve DflVE.

The upper parts of the liquid solubility curve, lying above
its intersection with the liquidus, are without importance as
regards the constitution of the solidified alloys, but have a
physico-chemical interest of their own. Methods which
involve rapid cooling of the liquid and mechanical separation .
of the solidified layers are unsatisfactory, and any attempts to
determine the course of the curve, and to fix the temperature
of the critical point, must involve the separation of the layers
while still liquid by suitable pipetting or tapping.^

' G. H. Clamer, J. Franklin Inst, 1903, 166, 49.
' Such a curve has been observed in mixtures of naphthalene with
w-dinitrobenzene. R. Kremann, Monatsh., 1904, 25, 1271.
^ See p. 90.

314

METALLOGRAPHY

4. Transformations of Solid Phases. — The horizontal lines
representing polymorphic changes in solid phases may or may
not intersect the liquidus. In the former case, a change takes
place at a certain temperature in the nature of the solid phase
in equilibrium with the liquid, and this involves a discon-
tinuity of direction in the liquidus curve. The effect on the
liquidus is therefore the same as that produced by a compound
decomposing below its melting-point, and it becomes necessary
to find a means of distinguishing between these two possi-
bilities. Such a case is illustrated in Fig. 97. The ascending

Fig. 97. — Compounds and polymorphic changes.

branch of the freezing-point curve presents two simil^ dis-
continuities, at r and / respectively, and alloys lying within
the respective limits of concentration have arrests on their
cooling curves, represented by the lines pq and rs. So far,
the thermal examination does not indicate whether compounds
are formed on cooling or not. The application of Tammann's
method, however, makes the distinction possible. If we
compare the duration of the arrests and construct arrest-curves,
we obtain such results as are shown in the figure. The arrest
/^has a maximum duration in the alloy containing 667 atomic
per cent, of B, indicating the formation of a compound ABj.

CONSTRUCTION OF EQUILIBRIUM DIAGRAM 315

The development of heat due to its formation is obviously
greatest in the alloy, the whole of which is converted on
cooling into the compound AB2, and falls off in mixtures
containing an excess of either A or B over that composition.
Had the arrest pq been due to a polymorphic change of B, its
duration would have increased from / onwards, being greatest
in the cooling curve of the pure metal B.

The arrest rs is represented as having its maximum dura-
tion at the same composition, indicating that it is due to a
polymorphic change in the solid compound ABg. Had it
been due to a reaction between AB3 and the liquid forming a
second compound, say AB, its maximum would have occurred
at a different composition— in the case last assumed at 50
atomic per cent. Only one of the many possibilities is repre-
sented in the figure, but the application of the method to other
cases is easy.

A polymorphic change occurring below the solidus may be
represented by a horizontal or by an inclined line. If by the
former, that is if it occurs at the same temperature throughout
its whole range, the substance undergoing the change, whether
element or compound, occurs in all the alloys in a free state,
and not as a solid solution. The maximum duration of the
arrest occurs at the composition of the constituent which
undergoes change. The arrest at rs (Fig. 97) is of this
kind.

An inclined line may represent a polymorphic change in
a solid solution, since the presence of a dissolved substance in
a solid alters transformation temperatures as the presence of a
dissolved substance in a liquid alters freezing temperatures.
Instances have been given in Chapter III. The method of
arrest-times is inapplicable to changes which take place at
temperatures varying with the composition, the temperatures
at which the arrests take place are therefore plotted on the
diagram, and other means must be adopted to determine the
nature of the constituent undergoing change.

Polymorphic changes during cooling are very liable to be
masked by undercooling, as the high viscosity of the solid
hinders molecular rearrangement, and the new phase, which is

3i6 METALLOGRAPHY

necessary to initiate change in the metastable region, is not
present when the transformation temperature is reached. We
are as yet ignorant of the interval which separates the meta-
stable limit from the transformation temperature in solids.
This limit once passed, and the labile region entered, the
presence of the new phase is not necessary to initiate the
change, but the viscosity is such that if only a short time is
allowed for cooling through the critical range, the transforma-
tion may be wholly are partly suppressed.

It has been urged ^ that transformations in the solid state
should be investigated by means of heating curves rather
than of cooling curves, on the ground that the transformation
point is more readily overstepped in descending than in ascend-
ing. Actually, overheating does occur to some extent, and
the readings obtained from heating curves are liable to give
high values for the transformation temperatures. Both heating
and cooling curves should be taken in accurate work ; if they
indicate transformations at the same temperatures, their indi-
cations are certainly trustworthy; should there be a small
difference in the observations, the mean of the ascending and
descending readings may be taken. An alloy to be examined
for polymorphic changes should be first cooled as rapidly as
possible to ensure a fine crystallization, and then reheated to
the temperature of transformation.

The thermal method does not necessarily record every
transformation which takes place, as it is possible for a poly-
morphic change to occur with very little change in the energy-
content of the system. In such a case, however, there must
be a change of volume, and the transformation may there-
fore be detected dilatometrically, as described in Chapter
XII.

The important class of polymorphic changes which are
marked by an alteration in the magnetic properties are in-
vestigated by the methods described in Chapter XII. If the
magnetic change is found to occur at the same temperature
throughout a series of alloys, it is evidence that the magnetic
metal is present throughout the entire series in the free state.

' B. E. Curry, J. Physical Chem., 1907, 11, 425.

CONSTRUCTION OF EQUILIBRIUM DIAGRAM 317

This is the case with the alloys of gold and nickel,' the tempe-
rature at which nickel becomes non-magnetic being unaltered
by alloying with gold.

On the other hand, in a series of solid solutions, the
magnetic change takes place at temperatures varying with the
composition, as in the alloys of cobalt and nickel,^ the mag-
netic transformation curve of which resembles the liquidus,
although having a different slope.

The assistance of the microscope in determining trans-
formation points is essential, whether these have been recorded
as the results of thermal, dilatometric, or magnetic observations.
It is possible for a constituent to undergo polymorphic change
without any change being produced in the micro-structure, but
such cases are exceptional, and are probably often due to the
insufficiency of our means of developing structure. The
occurrence of magnetic change, for instancCj must, on our
present assumptions as to the molecular constitution of solids,
be accompanied by some alteration in the ultimate arrange-
ment of the molecules, which it should be possible to detect
by suitably delicate means. The exact observation of the
form and orientation of the minute etching-pits produced by
the action of reagents may render it possible to distinguish
crystalline arrangements which appear identical under the
comparatively coarse etching treatment to which the crystals
are commonly subjected. There is much room for progress
in this direction, the etching of apparently homogeneous
structural constituents having by no means received the
attention it deserves.''

The procedure in seeking for transformations by the
microscopical method consists in quenching specimens of each
alloy from several known temperatures, with the object of
preserving in a metastable or labile state, the phase or phases
constituting the alloy at the quenching temperature. If
cooling curves of the alloys have been taken previously,

' M. Levin, Zeitsch. anorg. Cliem., 1905, 45, 238.
" W. Guertler and G. Tammann, ibid., 1904, 42, 353.
' See F. Osmond and G. Cartaud, on the crystallography of iron, J.
Iron Steel Inst., 1906, iii. 444.

3i8

METALLOGRAPHY

these afford guidance as to the most suitable temperatures
from which to quench the specimens ; in the absence of any
knowledge as to the existence or position of thermal arrests,
the proper temperatures must be found by trial. The earliest
and most important investigation of this kind, the study of the
copper-tin alloys by Heycock and Neville, already referred to
more than once, was carried out entirely by the microscopical

CUy

1000 ■

\\

900
BOO

700

\ ^a

7

.\>

•^

eoo

A

(

SCO

a

\

Cwj Sw + Tiq. ^v

\

Of+ J

+

300

\ Sn

zoo

-

/O 20 30 4-0 SO 60 70 SO SO lOO

Atom, /o S^l^'

Fig.

-Incomplete diagram of copper and tin.

examination of quenched specimens, without the aid of
pyrometric observations of transformation arrests. The diagram,
modified in some minor details by later work,' is represented
in Fig. 98. It is not complete, the discrepancies between
the different observers being too great to permit the complete
demarcation of certain regions, notably that lying below the
y region. A few examples will make the method clear. A

' E. S. Shepherd and E. Blough, J. Physical Cliem., 1906, 10, 630 ;
F. Giolitti and G. Tavanti, Gazzelia, 1908, 88, ii. 209.

CONSTRUCTION OF EQUILIBRIUM DIAGRAM 319

certain number of specimens are allowed to cool from their
freezing-point to the ordinary temperature at a very slow rate,
even 24 hours being allowed for the cooling of a specimen
weighing 5-10 grams. These alloys, examined microscopi-
cally, are taken to represent mixtures in equilibrium. Other
alloys, having the same compositions, are cooled very slowly
down to a certain prearranged temperature, and are then
quenched as rapidly as possible.

The diagram is complicated, and observers differ as to the
exact changes taking place in alloys containing between 15
and 30 atomic per cent. Sn. In constructing the diagram,
most weight has been given to the quenching results of
Heycock and Neville, the boundaries of the respective fields
as determined by them being only altered when inconsistent
with equilibrium or when decisively shown to require modifica-
tion. Both the later researches mentioned were also con-
ducted with great care.

An alloy containing 9 atomic per cent. Sn, quenched at
700°, consists of primary crystals of a, surrounded by /3. The
|3 constituent is unstable below 480°, and a specimen cooled
slowly to 470° and then quenched, shows the a crystals
surrounded by a eutectoid mass of « and 8, produced by the
breaking up of /3.

5. Vertical Lines. — The limits of composition within which
the respective solid phases exist are represented by lines
which are vertical when we are dealing with pure components,
but more or less inclined when the phases consist of solid
solutions. The latter case has been considered, so far as
lines of considerable inclination are concerned, in the last
section. From a practical point of view, the boundaries at
atmospheric temperatures — that is, the points at which the
vertical or inclined lines cut the base of the diagram — are of
the greatest interest, as they indicate the nature of the phases
constituting an alloy of a given composition when cold. Three
methods are available for fixing these limits, namely, {a)
thermal analysis, (V) microscopical examination, {c) the observa-
tion of discontinuities in physical properties.

Jt has already been shown how the observation of the

320 METALLOGRAPHY

composition at which an arrest due to a chemical reaction
between crystals and mother- liquor, to the solidification of a
eutectic, or to the transformation of a solid phase disappears,
may be used to determine the limits of existence of a phase
at the temperature in question. The application of Tammann's
method in a complex case may be illustrated by an imaginary
example, in which round numbers are used for the sake of
convenience.' The equilibrium diagram is represented in
Fig. 99.

The freezing-point curve ABCDEFG, presents two eu-
tectic minima at B and E, a well-marked maximum at D, and
breaks on ascending branches at C and F. The points repre-
senting the end of solidification have been entered on the dia-
gram, and form the broken solidus curve, MBNPQRESTUG.
The duration of each arrest on the cooling curves has also
been recorded, and is plotted on the same diagram; the
time composition curves being dotted and inverted to avoid
overlapping. For example, alloys containing from o to 40
atomic per cent, of the metal Z, show an arrest at 400°, due
to solidification of the eutectic. The duration of this arrest
increases with the proportion of Z up to 22 per cent., and then
decreases. The arrest times form the curve, MiJN, becoming
zero at N. The curves, C/Q, Ri?S, ind,F/U are obtained in
the same way.

The evidence for the existence of a compound at the point
D is as follows : — ■

1. An alloy containing 50 atomic per cent, of the metal Z
freezes completely at 750°, the temperature remaining constant
until the whole is solid.

2. The development of heat at 500°, observed in alloys
containing 35 to 50 atomic per cent. Z, disappears at the latter
composition, as indicated by the curve C/Q.

3. The eutectic arrest at 600°, observed in alloys contain-
ing 50 to 75 atomic per cent, also becomes zero at the same
composition, as indicated by the line R^S.

4. The diagram also indicates an arrest at 300°, observed

' This example is taken from the author's paper in J. Inst. Metals,
1909, i. 227.

CONSTRUCTION OF EQUILIBRIUM DIAGRAM 331

in alloys containing 40 to 75 atomic per cent. Z. This is due
to a polymorphic change in the solid state, the /3 modification
stable at high temperatures passing into an a. modification on
cooling past 300°. The maximum development of heat due

o 10

Fig. 99. — Application of Tammann's method to a complex system.

to this cause is found at 50 atomic per cent., as indicated by
the curve V^W.

All these facts point to the existence of a compound
having the formula YZ.

The diagram indicates the presence of two other com-
pounds, causing breaks in the freezing-point curve at C and F,

T.P.C. V

322 METALLOGRAPHY

but not giving rise to maxima as at D. To take the point C
first :—

1. The arrest at 500° (curve C/Q) reaches a maximum at
40 atomic per cent. Z. The arrest is due to the reaction of the
crystals of the compound YZ, which have separated at higher
temperatures, with a part of the still liquid alloy.

2. The eutectic arrest at 400° (curve M^N) and the
transformation arrest at 300° (curve V^/W), reach zero at the
same composition, on the line PNV.

These facts point to the existence of a compound of the
formula Y3Z2.

It will be seen that the composition of the compound
could not have been obtained by an inspection of the freezing-
point curve alone. The break at C does not coincide with its
composition, but occurs earlier, at 35 atomic per cent. This
approaches more nearly to the formula YgZ. The absence of
any determinations of the time of arrest prior to Tammann's
researches is the cause of many of the erroneous formulas for
inter- metallic compounds found in the literature.^

Similarly, the point F represents the formation of a third
compound from crystals and fluid alloy. The maximum time
of reaction is found by the curve F/U to occur at 80 atomic
per cent. Z, corresponding with the formula YZ4. The time
curves R«S and V(/W, however, do not extend to 80 atomic
per cent. Z, but reach zero at a lower concentration, namely,
75 per cent. This indicates that the compound YZ is
capable of forming solid solutions (mixed crystals), with an
additional 5 per cent, of Y. In the same way, the form of the
curve, Yi\3, points to the retention of as much as 5 atomic per
cent, of Y by the metal Z in a state of solid solution.

The imaginary diagram, constructed for the sake of round
numbers, thus demonstrates the existence of three compounds
of Y and Z, namely, Y3Z.2, YZ, and YZ4, of which only the
second can be melted without decomposition, the first and
third decomposing below their melting-points into a liquid
alloy and crystals of YZ, and of a saturated solid solution of

' The physical methods described in Chapter XII. provide further means
of determining the composition of intermetallic compounds.

CONSTRUCTION OF EQUILIBRIUM DIAGRAM 323

Y arid Z respectively. The compound YZ exists in an a
and a /8 modification, having a transition point at 300°, The
interpretation of any real diagram is based on exactly the
same principles.

Whether the limits of existence of the solid phases at
atmospheric temperature correspond with those determined at
higher temperatures is best ascertained microscopically. Re-
ferring again to the diagram (Fig. 99), we see that slowly
cooled alloys containing 0-40 per cent. Z consist of two solid
phases, namely, Y and Y3Z2. Between 40 and 50 per cent.,
they consist of YjZa and the a. modification of YZ. From 50
to 75 per cent., the solid phases are aYZ, and the solid solu-
tion I, containing 75 atomic per cent. Z. Alloys of 75-80
per cent. Z are homogeneous, consisting only of the solid
solution I, which changes progressively in composition within
these two limits. From 80 to 95 per cent, there are again
two phases, the compound YZ4, which is the limiting member
of the series of solid solutions I, and the saturated solid
solution II, containing 95 per cent. Z. Lastly, alloys con-
taining 95-100 per cent. Z consist again of a single solid
solution, II.

For micrographic purposes, we may subdivide this series
somewhat further by considering the eutectic as a separate con-
stituent. Although a complex of two phases, the structure of
the eutectic is such as to give it a distinct microscopical in-
dividuality. Under a low magnification, it may appear to be
homogeneous ; and even when its heterogeneous character is
evident, it is clearly distinguished from the crystals of the p&ase
in excess. We may therefore break up the region between
o and 40 per cent. Z into two parts, one extending to 23 per
cent. Z, in which crystals of Y are associated with the eutectic,
and one from 22 to 40 per cent., in which crystals of Y3Z2 and
the eutectic occur together. The region from 50 to 75 per
cent. Z is similarly divided at 62"5 per cent.

The structure of alloys in a state of equilibrium is most
conveniently represented by a diagram of the kind devised by
Saiiveur.' Such a diagram, constructed for the series in

' See p. 28.

324

METALLOGRAPHY

question, is shown in Fig. loo. The relative volume of each
constituent in a given alloy, and, consequently, the relative areas
exposed in a representative section, are easily read off from
such a diagram. The only alloys which can present a homo-
geneous structure are those containing exactly 43 per cent, or
50 per cent. Z, and those containing between 75 and 80 per
cent, or between 95 and 100 per cent. Z. In addition to these,
the two allo.ys containing 22 and 6 2 -5 per cent. Z respectively
present only a single micrographic constituent, a binary eutectic.
The structure of any other alloy of the series may be ascer-
tained from the diagram by following the ordinate correspond-
ing with its composition upwards until a diagonal line is
intersected. A horizontal line drawn through the intersection
gives, on the vertical scale, the relative areas of the two con-
stituents whose fields meet along that diagonal.

eo so wo

AtomUi%Z.

The proportions may be checked by planimetric measure-
ments of the alloys, and, conversely, the constitutional diagram
being known, planimetric measurements may be employed as
a means of analysis, a horizontal line being drawn through the
proportion of the two constituents found, and a perpendicular
dropped from its intersection with the corresponding diagonal
on to the axis of composition. Planimetric analysis has been
used as a means of estimating the carbon in soft steels,^ and
the oxygen in copper,^ and has been further extended to a

' A. Sauveur, Metallographist, 1898, 1, 27.

' H. O. Hofmann, C. F. Green, and R. B. Yerxa, Trans. Amer. Inst.
Min. Eng., 1904, 34, 671.

CONSTRUCTION OF EQUILIBRIUM DIAGRAM 325

number of copper alloys.* For the practical application of the
method, the image of the sectionj the structure having been
suitably developed, is projected on to a screen, and the outlines
are traced with a pencil. A border is then ruled, and the area
of one constituent measured with the planimeter and compared
with the whole area enclosed within the border. If the outlines
are very intricate, the planimeter becomes untrustworthy, and
better results are obtained by ruling lines, dividing the flihole
surface into squares i X i cm., shading the areas of one of the
constituents, and estimating the relative proportions shaded
and unshaded in each square — an operation which is soon
accomplished with considerable accuracy. A projection Is to
be preferred to a photo-micrograph, on account of the larger
scale, and also because the elimination of unnecessary detail
makes it easier to follow the outlines.

If the section consists of primary crystals with a small
quantity of eutectic, the area of the latter is liable to be much
less than that indicated by the diagram, owing to the attraction
of the large crystals for the particles of similar nature in the
eutectic. In an examination of phosphor-coppers containing
less than the eutectic proportion of CugP, it was observed that
the area of the copper crystals was always greater than that
calculated,'^ so that the diagonal line in a diagram like Fig. loo
was replaced by a curved line convex upwards. No explana-
tion was offered, but the same fact had been observed inde-
pendently by Huntington and Desch {loc. cit.), who showed
that the error was due to the segregation of copper from the
eutectic, leaving a shell of copper phosphide surrounding the
copper crystals. This shell appears as a band in the section
(p. 204), and by measuring its area, calculating (from the
known composition of the eutectic) the amount of copper
which must have been removed by segregation in order to
produce it, and deducting this from the observed area of
copper, an accurate result is arrived at, as shown in the
following table : —

' A. K. Huntington and C. H. Desch, Trans. Faraday Soc, igo8,
4, 51.

' E. Ileyn and O. Bauer, Zeitscli. anorg. Chem., 1907, 62, 129.

326

ME TALLOGRAPHY

Area of

Corrected

Percent. Pby

Per cent. P by

Structure.

crystals.

area.

planimetric

analysis.

Per cent.

Per cent.

measurements.

Cu + eutectic

8o-o

67-8

275

273

51

42-0

36-4

5-30

S-o8

,, ,,

37-8

25-7

6-15

6-II

■» >)

37'o

360

5-38

5-45

24-0

193

6-65

6-55

)> I)

1 1 -2

6-8

7-60

7-88

All eutectic

. —

—

8-27

8-25

)j »»

—

—

8-27

8-50

CU3P + eutectic

lO'O

—

8-85

8-93

)» »i

lO'O

—

885

9-05

i6-6

—

9-28

9-64

.1 i>

zr8

—

9"So

9"59

i> n

450

—

io'9

10-30

"

So'o

~

II-2

1070

The results obtained with alloys containing about 10 per
cent. P are in some cases too high, apparently owing to under-
cooling.

Should the limiting concentrations found by microscopical
examination differ from those found at higher temperatures by
the thermal method, and the accuracy of the latter is not
doubted, it is probable that the lines assumed to be vertical are
really inclined. This is put to the test by quenching experi-
ments, as described above.

It has been shown in Chapters XII. and XIII. that the
more important physical properties of a series of alloys vary
with the composition in a continuous manner so long as the
solid phases remain the same, but that the appearance or dis-
appearance of a phase at a certain limit of composition is
frequently marked by a discontinuity. This fact may be
utilized to determine the limits of composition at which each
phase appears. The most important property from this point
of view, in spite of the possibilities of experimental error that
it involves, is the electromotive force. Next to it in im-
portance, and supplementing its indications, stands the electrical
conductivity, with which the hardness is closely associated.
The specific volume and other physical properties enumerated
can have only a subordinate importance from this point of

CONSTRUCTION OF EQUILIBRIUM DIAGRAM 327

view.. Only when the indications of one or more of these
methods have been applied to assist in the interpretation of the
microscopic structure can the constitution of the alloys be
regarded as conclusively established.

The construction of diagrams representing the equilibrium
of ternary systems differs only in complexity from that described,
and does not involve any differences of principle ; it is there-
fore unnecessary to give details, which must vary widely with
the character of the system studied.

CHAPTER XV

THE MOLECULAR CONDITION OF METALS IN ALLOYS
AND THE NATURE OF INTER-METALLIC COMPOUNDS

Alloys being regarded as solutions, the consideration of their
molecular conditions forms a part of the general theoretical
study of solutions. Whilst, however, the discussion of solu-
tions in which the solvent is water or a similar liquid has been
devoted mainly to dilute solutions, the solvent being in large
excess, such a limitation is not possible in regard to alloys.
The distinction between " solvent " and " solute " even becomes
meaningless when dealing with mixtures of substances so
closely allied to one another as are the metals, mixed in all
possible proportions. The laws of dilute solutions are only
applicable to a very small range of alloys at the two ends of
each series. Further^ no help is to be derived from theories of
electrolytic dissociation, since electrolytic conduction does not
occur in alloys, as has been proved by numerous experiments,
and it is therefore impossible to infer the behaviour of the
compounds which metals form with one another from an
analogy with salts.

From the point of view of the molecular condition in
solution, the alloys of mercury with other metals, the so-called
amalgams, have naturally received most attention, owing to the
fact that mercury and its dilute alloys are liquid at the ordi-
nary temperature, so that freezing-point determinations may
be carried out in an ordinary Beckmann's apparatus, and
measurements of vapour pressure and surface tension may also
be made — a proceeding which involves great experimental
difficulties if applied to other metals. The amalgams have

328

THE CONDITION OF METALS IN ALLOYS 329

also been much investigated from the point of view of their
electrolytic potential, and of the velocity of diffusion within
them.

The Freezing-point Curve

In examining the molecular condition of dissolved metals
we make use of the atomic fall, which is obtained by dividing
the depression of freezing-point by the atomic percentage
of the added metal. Such a procedure is only legitimate so
long as the solution is dilute, that is, so long as the per-
centage of the second metal in the alloy is small, hence the
advantage of closely grouped freezing-point determinations in
the neighbourhood of the pure metals (p. 303).

The theoretical depression of freezing-point is given by
van 't Hoff's equation —

0-02T2

in which A is the depression due to i mol. of the solute in
100 mols. of solvent, T the absolute temperature of freezing,
and Q the latent heat of fusion. This formula allows the
molecular weight of the solute to be calculated from the
observations if the latent heat is known, or conversely,
the latent heat may be calculated if an assumption be first
made as to the molecular condition of the solute. The
formula has been applied in both ways. In the first place,
the molecular weight of most metals dissolved in mercury has
been found to be equal to their atomic weight,^ the theoretical
molecular fall being calculated as 2'i2°, and the mean value
obtained with several metals being 2 '12°. Van 't Hoff's
equation is, however, only applicable if the solid phase
separating on freezing is the pure solvent, in other cases the
atomic fall does not coincide with the molecular. Thus
cadmium, even in the smallest quantities, raises the freezing-
point of mercury. Tammann obtained similar results with
sodium as solvent, and these mixtures were independently

' G. Tammann, Zeitsch. physikal. C/iem,, 1889, 3, 441,

33°

METALLOGRAPHY

studied at the same time by Heycock and Neville/ who also
found that the dissolved metals behaved, in the majority of
cases, as if monatomic. In the case of gold in sodium, the
curve extending from pure sodium to the eutectic alloy,
containing 3-5 atomic per cent, of gold, is strictly rectilinear,
and gives the atomic fall 4'5. Similar results were obtained by
the same investigators, using as solvents tin,° bismuth, cadmium,
lead,' thallium,* zinc,' silver, and copper.* The general con-
clusion arrived at is that the state of metals in dilute solution
which do not form solid solutions is almost invariably that of
single atoms. The latent heats of fusion and calculated
molecular depressions of a few metals are collected in the
following table ' : —

Metal.

T.

Q-

A

Hg . . . .

234°

566 cal.

1-94°

Na . .

• 370-5

730

375

Sn. . .

505

1689

3-00

Bi . . .

546

2635

2-02

Cd . .

595

1535

4-6r

Pb. . .

600

nil

6-47

Zn. . .

692

1840

5-2

Ag

1234

2275

13-2

Cu .

J3S7

2646

13-9

Fe. . .

1778

1 120

56

rt . . .

1983

5295

148

The formula given only applies to very dilute solutions,
and cannot be employed to determine the form of the " ideal
curve.'' A more complete formula, extending over the whole
range of the freezing-point curve, was arrived at independently

' Trans. Chem. Soc, 1889, 58, 666.
= Ibid., 1890, S7, 376.
' Ibid., 1892, 61, 888.
* Ibid., 1894, 65, 31.
' Ibid., 1897, 71, 383.
" Phil. Trans., 1897, 189a, 25.

' The figures given have been recalculated from the most recent
available data.

THE CONDITION OF METALS IN ALLOYS 331

by I. Schroder^ and H. Le Chatelier," on the assumption that
the latent heat of fusion is unaltered by the addition of the
second metal. The modified equation assumes the form —

^«* = ?(T;-f)

in which x is the molecular concentration of the solvent metal,
that is, the fraction of a molecular weight of the solvent con-
tained in every molecular weight of the mixture. Assuming
the metals to be both monatomic in the alloy, x = the atomic
percentage of the solvent -4- 100. The ideal freezing-point
curve is then constructed by plotting the values oix from i to o
against the values of T obtained from the equation. The curve
thus constructed may have a point of inflection. The other
branch is constructed by applying the same process to the
freezing of the second metal. Heycock and Neville found
that the ideal curve thus constructed for alloys of silver and
copper agrees fairly closely, as regards the position of the
eutectic point, at which the branches intersect, with that
determined by experiment, although in this case solid solutions
are formed to a limited extent.

It has been shown ^ that by the introduction of corrections
for the variation of latent heat with composition, and for the
association of the solute in concentrated solutions, ideal curves
corresponding very closely with those obtained experimentally
may be drawn. The application of these corrections, however,
demands a previous knowledge of the experimental freezing-
point curve, so that the method cannot be used to predict the
form of the latter, although it may prove of value in explaining
abnormalities in certain instances.

' Zeitsch. fhysikal. Chetn., 1893, 11, 449.

^ Compt. rend., 1894, 118, 638. See also J. J. van Laar, Proc. k. Akad.
Wetensch. Amsterdam, 1903, 5, 424 ; 6, 21.

' D. Mazzotto, Nuovo Cim., 1908, [v.] 15, 401. The latent heat of
fusion of a number of alloys has been determined experimentally by
Mazzotto, Mem. Inst. Tj>mbardi, 1891, 16, I ; and by \V. Spring, Bull.
Acad. Sci. roy. Belg., 1886, [iii.] 11, 355.

332 METALLOGRAPHY

Vapour Pressure

The influence of dissolved metals on the vapour pressure of
mercury has been studied with the object of determining the
molecular weight of metals in the alloyed condition/ again
leading to the conclusion that the majority of metals are mon-
atomic in liquid amalgams. Only dilute solutions can be
employed, as at higher concentrations abnormal values are
obtained, owing, in many cases, to the formation of compounds
of mercury with the dissolved metal. The vapour-pressures
are measured directly, and controlled by measurement of the
vapour-pressure of pure mercury under precisely similar con-
ditions. The metals Li, Mg, Zn, Cd, Ga, Sn, Pb, Bi, Ag, and
Au are found by this method to be approximately monatomic.

Electromotive Force of Amalgams

The number of atoms in the molecule of the dissolved
metal may also be deduced from measurements of the E.M.F.
of concentration cells, in which a salt-solution is used as the
electrolyte, and the two electrodes are a concentrated and a
dilute solution of the one metal in the other respectively.
This method is only applicable, for experimental reasons, to
liquid amalgams, and many experiments have been made in
this direction. The E.M.F. of such a cell is given by the
formula ^ —

n C2

in which R is the gas constant, c^ and c^ the concentrations of
the metal in the two amalgams, T the absolute temperature,
and n the number of unit charges carried by one molecule of
the metal. If the metal is monatomic, n = the valency of the
metal ; if several atoms are associated to form a molecule, n is
correspondingly greater, and dividing it by the valency, «',
gives the number of atoms in a molecule. The value «/«' is

• \V. Ramsay, Trans. Chein. Soc, 1889, 55, 521.
' G. Meyer, Zeitsch. fhysikal. C/iein., 1891, ,7, 477.

THE CONDITION OF METALS IN ALLOYS 333

found to be sensibly equal to i in amalgams of zinc, cadmium,
lead, tin, copper, and sodium.'

On the other hand, it has been shown by Haber ^ that the
above conclusion is not strictly justified. The experimental
results prove that the dissolved metals are present in solution
either as single atoms or as compounds with mercury having
the formulae MHg„. Haber obtains, as a more complete form
of the above equation : —

E = RT/« - + m X o-oi474RT(<r, - c^

Within the limits of error of the experiments, it is impracticable
to distinguish between the two equations, and the results
obtained therefore prove that atoms of zinc, etc., are not
associated together in amalgams, but do not prove that they
are not combined with mercury to form compounds of the
general type MHg,„.

The objection raised by Haber to the conclusions from
electro-chemical data is extended to the other methods of
investigation by McPhail Smith,* according to whom the
freezing-point and vapour-pressure methods also only prove
that either single atoms or molecules MHg^ are present, The
question is really the same as that of the hydration of salts in
aqueous solutions. Freezing-point and similar methods enable
us to determine the molecular weight of the salt, but do not
indicate clearly whether, and to what extent, the salt is
hydrated. It is, in fact, far more difficult to determine how
far the molecules of the solute are combined with those of the
solvent than to determine the extent of association of the
solute.

In simple eutectiferous series, in which the freezing-point
curve shows no indication of the formation of compounds, we
may safely accept the conclusion arrived at by the three

' A. Scholler, Zeitsck. Elektrochem. , 1898, 3, 259; T. W. Richards
and G. Lewis, Zeitsch. fhysikal. Client., 1899, 28, i ; II. Fay and E. Norlh,
Amer. Cheni. J., 1901, 2S, 216.

^ F. Haber, Zdtsch. physikal. Client., 1902, 41, 399.

' G. McPhail Smith, Amer. Cliem. y., 1906, 36, 124 ; 1907, 38, 671.

334 METALLOGRAPHY

methods described above, that the dissolved metals are
monatomic. In series in which compounds occur, it remains
as yet an open question whether the same condition prevails,
or whether compounds, containing a single atom of the
dissolved metal in each molecule, are also present,- The only
experimental method which gives any indication of a definite
answer to this question is that of measuring the velocity of
diffusion in liquid metals. With the exception of some
isolated experiments with molten lead, tin, and bismuth,' such
measurements have been confined to mercury. Fick's law of
diffusion, according to which the quantity diffusing through a
given area in a given time is proportional to the rate of change
of concentration, is confirmed in all cases.^ The diffusion
constant k, which is expressed in square centimetres per day,
is given for a number of metals in the second column of the
table, the solvent being mercury,^ Since the constants calcu-
lated in this way refer to weights of metal in grams, it is
necessary to divide the value of k by the atomic weight of the
metal diffusing.* It is then found, on examining the atomic
diffusivity as a function of the atomic weight, that whilst the
metals Zn, Cd, Sn, and Pb, which do not form compounds
with mercury, fall on a smooth curve (Fig. loi), the metals of
the alkalis and alkaline earths, together with thallium, give
points falling on an entirely distinct curve (the lower in the
figure). These are metals known to form compounds with
mercury, and the conclusion is drawn that the lower rates of
diffusion observed for them are due to the fact that each atom
of the diffusing metal carries with it a number, at present
uncertain, of mercury atoms associated with it.

' W. C. Roberts-Austen, Phil. Trans., 1897, 187a, 383.

* F. Guthrie, Phil. Mag., 1883, [v.] 16, 321 ; W. Humphreys, Trans.
Chem. Soc, 1896, 69, 243, 1679. Both these experimenters placed the solid
metal in contact with the mercury, rendering exact calculation impossible.
See W. C. Roberts-Aiisten, Proc. Chem. Soc, 1896, 12, 219. In later
experiments, concentrated amalgams have been used instead of solid
metals.

' M. von Wogau, Ann. Physik., 1907, [iv.] 23, .345.

* G. McPhail Smith, Zeitsch. anorg. Chem., 1908, 58, 381,

THE CONDITION OF METALS IN ALLOYS 335

k = cm.=
day

*

atomic weight

Li

0-66

0-0939

Na

.

064

0-0278

K

0-S3

0-OI35

Hb

0-46

0-0054

Cs

0-45

0-0034

Ca

o'54

O-0I3S

Sr

0-47

0-0054

Ba

0-52

0-0038

Zn

2-i8

0-0333

Cd

I -45

0-0129

Tl

0-87

0-0043

Sn

I-S3

0-0128

Pb

•

1-50

0-0072

The form of the freezing-point curve in the neighbourhood
of a maximum may also be studied as a means of gaining
information respecting the behaviour of inter-metallic com-

izo

100

o 20 ■*(? 60 ao 100 120 i-^o. 160 lao zoo 220

Fig. ioi. — Atomic diffusivities of metals in mercury.

pounds in solution in liquid metals. The formation of an
inter-metallic compound has in the simplest case the eflfect of
breaking up the equilibrium diagram into two parts, each
representing a simple binary system, the components being,
in the one half, the metal A and the compound of A and B,

336 METALLOGRAPHY

and in the other the same compound and the metal B. The
entire diagram thus consists of two V-shaped curves. The
intersection of the two intermediate lines at a point, however,
would indicate that the compound melted without undergoing
any dissociation into its compoHents in the molten state.
Although it is claimed that such a condition has been realized
in the case of certain non-metallic mixtures,^ there are thermo-
dynamical grounds for supposing that such a condition is
incompatible with equilibrium, and that the compound must
undergo dissociation on melting, to however slight an extent.^
Under conditions of equilibrium, the tangent to the curve at
its highest point must be parallel to the axis of concentration.
The greater the flattening of the curve near this point, the
more completely we may suppose the compound to be dis-
sociated into its components. The attempt has been made to ,
calculate the extent of the dissociation by measuring the slope
of the curve on each side of the maximum,'' and applying the
principle of mass action. On each side of the maximum, the
compound is in the presence of one of its products of dis-
sociation, and approximate estimates have been made of the
relative proportions of the undissociated compound and of
the components at each temperature. The process involves
many assumptions, rendering its application to alloys very
hazardous. By adding an indifferent substance to the molten
compound, however, and measuring the depression of freezing-
point which it produces, a value may be obtained for the
atomic lowering of freezing-point of the compound, and this
may be applied to determine the extent of dissociation in the
binary system.'' It is therefore possible that a systematic

' The freezing-point curve of mixtures of methyl iodide and pyridine
has two lines intersecting at an apparently sharp angle. A. H. W. Aten,
Proc. k. Akad. Wetensch. Amsterdavi, 1904, 7, 468 ; Zeitsch. physikal.
Chem., 1905, 54, 127.

' II. A. Lorentz, Zeitsch. physikal. Chem., 1891, 7, 36 ; R. Ruer,
ibid., 1907, 59, i ; 1908, 64, 357.

^ R. Kremann, Monatsh., 1904, 25, 1215. Compare A. Findlay,
Trmis. Faraday. Soc, 1907, 3, 153.

' R. Kremann, loc. cil., compare W. Stoxitnhtcker, Zeitsch. fhysikal.
Chem., 1S92, 10, 183.

THE CONDITION OF METALS IN ALLOYS 337

investigation of ternary series may enable such calculations to
be made. Given a binary system, containing a compound AB
giving rise to a maximum in the freezing-point curve, and a
third metal which forms simple eutectiferous series with each
component of the binary series, observations of the slope of
the curve of mixtures of AB with C should give the necessary
data for estimating the dissociation of AB in the molten state.

The number of inter-metallic compounds already known
is large, and new investigations constantly result in further
additions to the list. When a report on the subject was drawn
up in 1900,* the compounds tabulated numbered 37. A
similar table drawn up in 1908,'' to which only compounds
were admitted the evidence in favour of whose existence was
considered satisfactory, contained the formulae of 109 com-
pounds. This number is certain to be very greatly increased
during the next few years.

It has been shown in the last chapter that the establishment
of the existence and composition of an inter-metallic com-
pound is a matter of considerable experimental difficulty,
especially when, as is often the case, the compound is capable
of forming solid solutions with one or both of its components."
Two errors of method have been responsible for the introduc-
tion into chemical literature of a very large number of
erroneous formulae purporting to represent definite compounds.
One of these is the attribution of a discontinuity of any
physical property in a series of alloys at a certain composition
to the existence of a compound having that composition. We
have seen that such discontinuities occur at the point of
appearance of a new phase, and that although their ultimate
cause may be chemical combination, the formula of the
compound cannot be directly inferred. The second error is
that of assigning a definite formula to any crystals of homo-
geneous appearance, isolated from cooled alloys either
mechanically or by treatment with chemical reagents. This

» F. H. Neville, "Report on the Compounds contained in Alloys,"
Brit. Assoc. Rep., Bradford, 1900.

= C. H. Desch, J. Inst. Metals, 1909, i. 227.
' See C. H. Desch, Interinetallic Compounds (London, 19 13).
T.P.C. Z

338 METALLOGRAPHY

method has also been criticized in Chapter XIII. In spite
of the progress which metallographic investigations have
made, it is not uncommon to find mixtures of metals described
as compounds on such insufficient grounds in chemical periodi-
cals of the present day.

One important aid in fixing the formulae of compounds
which is available to the organic or inorganic chemist is denied
to the metallographist. This is the application of the doctrine
of valency. Whilst the formula of an organic compound
which is inconsistent with the known valencies of the elements
contained in it may be immediately ruled out — even the
proposal to ' regard triphenylmethyl as containing tervalent
carbon had only a short life — and the same principle is applied
in the chemistry of inorganic compounds containing non-
metals, the inter-metallic compounds are impossible to bring
into the scheme of valency as generally understood. Such
formulae as NaHga, AuSn4, AugAl, and AugZHj do not accord
with any usual valency that can be assigned to the atoms
composing the molecules.

When one component of a compound has a semi-metallic
character, the proportions in which it combines with a metal
approach more nearly to those demanded by the respective
valencies than when both components are markedly metallic.
Thus antimonides and arsenides, even when possessing all the
properties of true alloys, have formulae in which the antimony
may be represented without difficulty as tervalent, as in —

AgsSb CujSb MnsSbi

AlSb Fe^Sba Na^Sb

CdgSbi! FeSba Tl^Sb

CoSbz MgsSba ZnjSba
CrSbj

The same valency may be retained for antimony in the follow-
ing antimonides by assuming the metals Co, Cr, and Ni to be
tervalent : —

CoSb CrSb NiSb

whilst the following formulae, apparently well established, are
less easily reconciled with the valency :—

THE CONDITION OF METALS IN ALLOYS 339

AuSba

Mn^Sb

NaSb

CdSb

Ni4Sb

ZnSb

CuoSb

Ni^Sba

The antimony appears to be quinquevalent in one of the com-
pounds with nickel.

It is of course possible that the formulae of some of the
inter-metallic compounds may require to be multiplied by a
factor, in which case it would become easier to assign valencies
to their components, but the evidence necessary to determine
the molecular weights of compounds in alloys is very scanty.

The more' nearly the two components approach one
another in chemical character, the less regular the formulae of
their compounds appear. Neither is it possible to predict
with any certainty, from the compounds formed by a metal A
with another B, the formulse of the compounds which A will
form with other metals belonging to the same group in the
periodic classification as B. A few general rules have indeed
been stated by Tammann,^ but these are subject to many
exceptions. Tammann's principal conclusions are —

1. Neighbouring elements in a natural small group do not
form compounds with one another. Iron, however, appears
to form a compound with nickel.

2. A given metal either combines with all the elements of
such a group or with none. It does not follow, however, that
these compounds will resemble one another in constitution.
Thus cadmium combines with gold, silver, and copper, but the
formulae of the compounds exhibit no correspondence —

CujCds AgaCdj AujCds

CusCd AgCd ? AuCda

AgCdj?

Gold combines with lead to form AuaPb and AuPba, but
neither copper nor silver combines with lead.

It has been suggested by Kurnakoff^ that Mendeldef's
rule, according to which the total valency of an element is the
sum of its valencies towards oxygen and hydrogen, and is

' Zeitsck. anorg. Ckeiii., 1906, 49, 113 ; 1907, 55, 289,
« Jbul., 1900, 23, 439.

340 ' METALLOGRAPHY

equal to 8, may be applicable to alloys, one of the metals
taking the place of hydrogen. This would allow the alkali
metals, which are univalent towards oxygen, to exhibit a
valency as high as 7 in inter-metallic compounds. Whilst the
number of atoms associated with one atom of the alkali metals
in their compounds with other metals is often high, as in the
following instances : —

KZn„

NaCds

CsHg,

NaZnii

NaHg4

CsHge

KCdu

Na^Pbo

KBi^

there is no evidence that such a rule applies.

If we assume that the metals only combine with one
another by means of their latent valencies,^ and regard the
antimonides as resembling the sulphides and silicides rather
than the true inter-metallic compounds, we no longer feel
surprise at the irregularities in the apparent valencies exhibited
by metals in alloys. The difficulty is, however, only thrown
back a stage, and not removed, and the full explanation of
these anomalies awaits the advent of a theory of valency
sufficiently comprehensive to account for " latent " and
" partial " valencies in a quantitative manner.

' See The 27ieoiy of Vateticy, by J. N. Friend, in Ihis series, p. 155.

CHAPTER XVI

THE PLASTIC DEFORMATION OF METALS AND ALLOYS

Metals and alloys exhibit a certain degree of extensibility
and compressibility. The application of a mechanical stress
tending to cause extension, compression, or change of form
results, in the first place, in the production of an elastic strain,
that is, the solid undergoes a change of dimensions or of form
during the application of the stress, returning to its original
condition after the removal of the applied force. During this
stage of perfect elasticity the strain produced is strictly pro-
portional to the acting stress (Hooke's law) ; but when the
stress exceeds a certain limit, this proportionality no longer
prevails, and a solid subjected to tension, for example,
elongates more rapidly than corresponds with its behaviour
during the elastic stage. On removal of the tractive force, the
solid does not return completely to its original condition, but
retains a small permanent elongation. With further increases
of stress, the permanent changes become more and more
considerable relatively to the elastic changes. The compressi-
bility of a metal is generally diminished by alloying.^

Strains produced within the elastic range do not result in
any visible change of structure. The alterations in certain-
properties of metals and alloys under elastic strain, such as the
electrical conductivity and thermo-electric power, are of con-
siderable interest, but the experimental difficulties have
hitherto stood in the way of obtaining results which admit
of expression in a general form.

The limiting stress of the elastic range, or " elastic limit,"
a magnitude of high technical importance in the investigation

' S. Lussana, Nunw Cini., 1910, [v.] 19, i. 182.
34 1

342 METALLOGRAPHY

of structural materials, is variously taken as the point at which
the strain ceases to be proportional to the stress, or as the
point at which the permanent change exceeds a certain
arbitrary small amount, depending on the sensitiveness of the
testing apparatus employed. The two definitions are not
quite equivalent.

The permanent alterations of volume or form resulting
from the application of a stress in excess of the elastic limit
are an object of study of the metallographist, as they are
accompanied by visible changes in the macroscopic or micro-
scopic structure, the character of which gives important
information as to the internal arrangement of the component
particles of the solid. Such changes are intimately connected
with the crystalline structure of the metal or alloy, and may be
employed as a means of distinguishing between closely related
crystalline forms, as in certain cases to be discussed below.
Their character varies with the nature as well as with the
amount of the applied stress.

Pure compression, unaccompanied by any shearing stress,
does not enter into consideration. The changes of density
produced have been alluded to in Chapter XII., and are pro-
bably due in all cases to a lack of homogeneity in the speci-
mens examined. A homogeneous solid is perfectly elastic
under compression unaccompanied by change of shape.
Deformation by stresses in excess of the elastic limit, however,
may be accompanied by alterations in volume which persist
after the removal of the stress^ so that the deformed metal has
a different density from the metal in a state of equilibrium. It
has been shown in Chapter XII. that the density of the strained
metal is generally less than that of the unstrained, bismuth
being a conspicuous exception.

The metallographist is concerned with two distinct pro-
blems bearing on the non-elastic deformation of metals and
alloys, namely, the mechanism of deformation, and the nature
of the new physical condition indicated by the change in pro-
perties of a strained when compared with an unstrained metal.

Regarded in the mass, without reference to the microscopic
structure, a metal exhibits a certain degree of plasticity, yielding

PLASTIC DEFORMATION OF METALS 343

under tension, compression, torsion or bending, like pitch,
and apparently only differing from the latter substance in the
extent of its resistance to deformation. The resistance may
vary from, that of glass-hard steel or speculum metal, which
breaks without appreciable deformation, to that of sodium or
lead, which may be squeezed or moulded with ease. The
plasticity of metals and alloys is generally under-estimated, and
is only revealed to its full extent by experiments designed for
the purpose. The fact that metals could be made to flow
through openings by pressure, and to behave in other respects
like fluids of high viscosity, was first demonstrated by Tresca,^
who made evident the lines of flow in the mass by the device
of employing thin superposed sheets of metal instead of solid
blocks. Tresca's experiments have often been repeated under
different conditions ; and the results obtained by Spring, Tam-
mann, and others show that the property of flowing through an
opening, and even of forming a continuous wire, is not confined
to metals commonly recognized as plastic, but is shared by such
brittle metals as bismuth and antimony. The order of plasticity
revealed by some of the most familiar metals is^

K, Na, Pb, Tl, Sn, Bi, Cd, Zn, Sb.

The fact that bismuth is more readily pressed into the form of
wire than cadmium is remarkable, but in other respects the
series is in accordance with the general behaviour of the
metals.

This property of metals and alloys, of flowing like a viscous
fluid when a sufficient pressure is applied, has been utilized in
the technical production of tubes and rods. Lead and alloys
of lead with tin or antimony are made into tubes by forcing
the mass through a die having an annular opening, at a tempe-
rature sufficiently high to cause a marked increase of plasticity,
but yet well below the melting-point. Alloys of copper and
zinc are similarly " squirted " at a temperature below the
melting-point, the process being only applicable to alloys con-
taining the j3 constituent. The plastic yielding of metals

» Cmnpt. rend., 1864, 69, 754 ; 1865, 60, 398 ; 1867, 64, 809 ; i868,
66, 263; 1869, 68, 1197 ; 1870, 70, 27, 288, 368.

344 METALLOGRAPHY

during rolling, forging, wire-drawing, etc., is essentially of the
same character.

The first metallographic problem named above is that of
the internal mechanism of the plastic deformation. The super-
ficial resemblance between the plastic behaviour of a crystalline
solid and of an amorphous substance like pitch or glue is not
due to an identity of the processes in the two cases. The flow
of pitch is in every respect that of a liquid of high viscosity,
but microscopical examination shows that the phenomena in
crystals are of a quite different order. The problem was first
studied in connection with the behaviour of large masses of ice.
The indisputable fact that glaciers move like fluids of high
viscosity was explained by Forbes ^ as being due to the plastic
flow of ice-crystals under the action of slowly applied stresses.
In opposition to this, it was maintained by Tyndall ' that the
flow was due to regelation, that is, that the pressure on the ice,
by lowering the melting-point, caused the production of a
small quantity of liquid water, allowing the grains of ice to roll
or slide over one another, freezing and consequent re-consoli-
dation of the mass taking place as soon as the stress was
relieved. It will be seen that this explanation is necessarily
confined to substances like ice, the melting-point of which is
lowered by pressure. Other crystalline solids, however, behave
like glacier ice. The well-known " creeping " of lead sheets on
a sloping roof is of a similar nature, the weight of the lead
causing it to flow downwards like a fluid of high viscosity. Ice
is plastic at temperatures below those at which liquid water can
be formed, and subsequent research has shown that plasticity
is quite independent of regelation. As regards glaciers, the
cause of flow was for long a subject of controversy, but the
existence of true plasticity in the mass has now been placed
beyond doubt.'

' J. D. Forbes, Travels through the Alps of the Savoy, 184S > Proc.
Roy. Soc. £iUh., 1858, 4, 103; Occasional Papers on the Theory of Glaciers,
1859.

« J. Tyndall and T. H. Huxley, Phil. Trans., 1857, 147, 327 ; J.
Tyndall, Glaciers of the Alps, i860.

' See J. Ruskin, Deucalion, 1879, Vol. XXVI. of Collected Edition,
1906.

PLASTIC DEFORMATION OF METALS 345

Many crystals are capable of being deformed without losing
their crystalline character, from the oleates, which are so soft
that two crystals brought into contact at a point unite to form
a single one,^ or potassium manganous chloride, a crystal of
which may be pressed into a lenticular shape by the fingers
without being cracked or broken,^ to the apparently rigid
crystals of calcite or rock-salt. Even these may be deformed
to an extraordinary extent if certain precautions are taken. If
enclosed in a metal tube, tightly packed with a powder, the
tube may be bent or hammered so as to produce a great
plastic deformation of the enclosed crystal without inducing
fracture.' Twinning planes are developed abundantly by such
treatment, and the well-known planes of repeated twinning in
the crystal grains of saccharoid marble are certainly due to
flow under pressure, and may be reproduced artificially.''

Twinning planes are also of very frequent occurrence in
metals and alloys which have been subjected to mechanical
deformation. Microscopical examination has shown, however,
that the mechanism of yielding under mechanical stress is
for the most part of a finer character than twinning, and that
displacement of the particles constituting a crystal takes place
on a smaller scale, producing changes of structure which are
less obvious but more important than the comparatively gross
changes involved in twinning. If a piece of metal with a
smooth, polished surface is bent or stretched, a number of fine
lines make their appearance, running parallel to each other
over the area of a single grain, but generally varying in direc-
tion from one grain to another. The crystalline structure of
the metal is in this way made evident without etching, as the
boundaries of the crystal grains are revealed by the abrupt
changes in direction of the fine lines. The general effect is

' O. Lehmann, Flussige Krystalk, Leipzig, 1904.

' O. Miigge, Neues Jahrb. Min., 1889, i. 159.

= G. A. Daubree, Geologic expinnuntale, Paris, 1879 5 Kick, Zeitsch.
Ver. deut. Ing., 1890, 11 ; 1892, 919.

■• F. D. Adams and J. H. Nicolson, Phil. Trans., 1909, 195a, 363.
Earlier experiments are due to J. Hall, Trans. Roy. Soc. Edin., 1805,
6, 71.

346 METALLOGRAPHY

seen in Plate XI., A, which represents a smooth surface of lead,
cast in contact with glass, and lightly bent. The photo-micro-
graph shows the junction of three grains, and the lines, which
are not perfectly straight, are seen to be parallel within each
individual grain. It is not easy, at first sight, to recognize the
true character of these lines ; they are not cracks or ridges.
Under oblique illumination, lines having any given direction
are found to be visible only when the incident light falls on
them from a particular direction, so that they alternately appear
and disappear when the stage of the microscope is rotated.
The lines are therefore shown to be steps, and the name of
" slip-bands " has been given to them, as best expressing their
character.^ The height of each step is exceedingly small, but
it has nevertheless been found possible to give a direct proof
of its nature by means of a section perpendicular to the strained
surface. Any attempt to render fine detail visible when occur-
ring at the edge of a section fails owing to the unavoidable
rounding of the edge during polishing. This difficulty has been
overcome by the device of depositing a metal electrolytically
on the surface to be studied, so that on cutting through the
compound mass the original boundary appears in the middle
of the section instead of at the edge, and is thus protected from
the rounding action.^ For example, in the examination of slip-
bands in iron or steel, the surface on which the slip-bands have
been developed by straining is first coated with a thin layer of
copper by deposition from a cyanide solution (in order to avoid
any attack on the iron), a thick layer of copper being then
deposited from an acid solution. A section cut perpendicularly
to the original surface shows the required detail at the boundary
of the iron and copper. In this way Rosenhain has been able
to show very clearly the stepped character of a surface crossed
by slip-bands.

The formation of slip-bands takes place in the following
way. The regular orientation of the particles in a crystal
causes slipping of the particles over one another to occur

' T- A. Ewing and \V. Rosenhain, Diil. Trans., 1889, 193a, 353.
^ W. Rosenhain, Proc. Roy. Soc, 1905, 74, 557 ; J. Iron Steel Itist.,
1904, i. 335; 1906, ii. 189.

A. Strained surface of lead, showing slip-bands.

/ ( -^

B. Pearlite in steel, x 300.
PLATE XI.

[To face page 346.

PLASTIC DEFORMATION OF METALS 347

more readily in certain directions than in others, giving
rise to systems of " gliding-planes." Fig. 102 represents dia-
grammatically the structure of a crystal, and it is evident that
a motion of translation in the direction of the horizontal line
may occur without any disturbance of the crystalline structure.

Fig. 102. — Translation along a gliding plane.

Translation may take place along that plane only, or along a
number of planes parallel with one another. A crystal may
possess several distinct systems of gliding-planes, the number
depending on the class to which it belongs. Slip-bands are
produced by successive small slips along gliding-planes of the
same system. In Fig. 103, a represents a polished surface

a

Fig. 103.-

-Formation of slip-bands.

before straining. Portions of three crystal grains are shown.
The condition of the metal after straining is represented at b.
Slipping has taken place along the gliding-planes, with the
consequent production of inequalities of level. It is these steps
on the surface which are known as slip-bands. The pheno-
menon exactly resembles the production of repeated faults, or
" step-faults," in geological strata.

As every crystal possesses several systems of cleavage or
gliding planes, slip-bands may be developed in several direc-
tions. The first effect of a moderate degree of straining on a
crystal grain is usually to develop slip-bands in a single direction
only. As the stress becomes more severe, or as its direction
changes in the course of the redistribution of stress due to

348 METALLOGR^PHV

local yielding, new systems of slip-bands appear, and several
intersecting systems of lines are consequently observed on the
surface of the specimen. Under high magnifications, the cross-
ing of two slip-bandsis seen to be marked by an abrupt disloca-
tion, a fact which points clearly to their stepped character.

The plastic yielding of crystalline solids under stress is
therefore due to slipping along innumerable gliding-planes
without, in the first instance, any disturbance of the crystalline
structure other than the translatory displacement of certain
layers of particles relatively to others. It has been maintained
by Heyn and others that a certain amount of plastic deforma-
tion takes place without the production of slip-bands, the metal
yielding without alteration of structure ; but Rosenhain has been
able to prove by measurement that areas lying between slip-
bands do not undergo change of form. A crystal grain may
often be observed to remain free from bands when the section
in which it occurs is deformed^ but in such a case the dimen-
sions of the crystal remain unchanged. The observations of
O. Lehmann on protocatechuic acid indicate very distinctly
tliat the plastic yielding, even of soft crystals, only takes place
by slipping along a large number of gliding planes.

If the force applied to the solid is one of tension, producing
elongation and lateral contraction, the component crystal grains
are themselves elongated, becoming more or less spindle-shaped,
so that a longitudinal section of the strained specimen has a
very characteristic appearance under the microscope. A
transverse section shows rounded polygonal grains, smaller in
size than in the unstrained metal. When the elongation is
very great, as in the drawing-down of wire through dies, the
individual grains may be lengthened into fibres, and even
divided, forming parallel bundles.' Severe mechanical strain-
ing, as in hammering or rolling, may result in the grains being
actually driven into and through one another, so that the
structure becomes minute and confused ; it is therefore difficult
to draw conclusions as to crystalline character and composition
from the microscopical examination of such specimens, except
after thermal treatment causing recrystallization.

' E. Heyn, Zeilsch. Ver. dtut. Ing., 1900,

PLASTIC DEFORMATION OF METALS 349

As the formation of slip-bands in the manner described
does not involve any change in the internal structure of the
metal, all the motions being those of simple translation, etching
removes all traces of the bands from the surface. Any bands
remaining after etching must be of a different character. Per-
sistent bands are indeed frequently observed in strained metals.
In many cases these represent planes of repeated twinning, and
such planes may occur so closely as to resemble groups of slip-
bands. In addition to these, a piece of metal sufficiently large
to be made up of many crystal grains may show after straining
lines which are apparently independent of the crystalline
structure. Such lines, which form systems of curves bearing a
definite and calculable relation to the direction of the stress,
occur both in crystalline and in amorphous substances, and
have the same general form in both cases.^ Nevertheless, their
minute structure is probably different in the case of crystalline
solids. Whilst the form of the curves represents the distribu-
tion of stress, they are probably made up, in the case of metals,
of short cleavages approximately coinciding with the direction
of the curve, the change from one cleavage to another being
discontinuous, so that each curve, instead of being smooth, is
stepped.

The different systems of lines developed by strain may be
studied in isolated crystals. Whatever lines appear under
such circumstances necessarily bear an intimate relation to
the crystalline structure. The only metal which has been
examined in any great detail from this point of view is iron, in
which both the slip)-bands and twinning planes may be well
studied.^ The behaviour of different faces of a crystal may be
compared by studying the " percussion figures," or groups of
curves and cleavage lines produced by striking the face to be
examined with a pointed instrument. Such percussion figures

' F. Osmond, C. Fremont, and G. Cartaud, Rev. de Mitallurgie, 1904,
1, II ; A. Mallock, Proc. Roy. Soc, 1909, 82a, 26 ; see also C. Fremont,
Bull. Soc. {J Encouragement, 1896, [v.] 1, 1218 ; F. Rogers, Riv. de Metal-
lurgie, 1906, 3, 518 ; G. H. Gulliver, Proc. Inst. Meek. Eng., 1905, 141.

= F. Osmond and C. Fremont, Rev. de Metallurgie, 1905, 8, 801 ;
F. Osmond and G. Cartaud, ibid., 1906, 3, 653 ; J. Iron Steel Inst., 1906,
iii. 144.,

350 METALLOGRAPHY

have been utilized in distinguishing between the different forms
in which iron crystallizes.'

Fatigue

It is well known that materials subjected to frequent alter-
nations of stress become " fatigued," the strength diminishing
after the alternations have continued for some time. A metal
subjected to rapid alternations of stress may break, even when
the stress has never exceeded the known elastic limit of the
material. A steel shaft in a factory, supported by brackets and
carrying pulleys with belts, is subjected, not only to a more or
less constant torsional stress, but to bending stresses which are
varying in direction within the shaft at every moment during
its revolution. It becomes important to discover the nature
of the fatigue observed in such structures, and to determine
the permissible limit of stress in each material. The stresses
applied may also consist of alternate tension and compression,
as in the connecting-rod of a locomotive, and many other
varieties of stress are possible. Metals of high elastic limit
are, in general, the best able to withstand repeated alternations
of stress, a typical example being seen in the hard steel of
which watch-springs are made.

Several different forms of machine have been employed in
the experimental study of fatigue. In one of the most con-
venient, due originally to VVohler, a cylindrical rod of the
material is made to project from the end of a revolving shaft,
the overhanging end being loaded by means of a ring slipped
over it, bearing a weight or forming the upper attachment of a
spring balance. The stress is in this case a bending one,
varying rapidly and continuously within the metal, as in the
factory shaft referred to above. By polishing a portion of the
surface of the test-piece, and examining from time to time
under the microscope, the mechanism of fatigue has been very
completely studied.^

' F. Osmond and G. Cartaud, Rev. de Metallurgie, 1905, 2, Sir,
and loc. at,

' J. A. Ewing and J. C. W. Humfrey, PAil. Trans., 1902, 200a, 241.

PLASTIC DEFORMATION OF METALS 351

Even although the stress, calculated on the specimen as a
whole, may be well below the elastic limit, it may not be
uniformly distributed throughout the mass, so that certain
crystal grains are more severely stressed than others. Slip-
bands make their appearance in these grains after a certain
number of reversals, and their number increases as the test is
continued. The slipping which takes place leads to a re-
distribution of stress among the crystal grains, and after a time
a larger number of grains are found to exhibit slip-bands.
Some of the slip-bands develop into cracks, and a crack once
started propagates itself rapidly, through causing a local in-
tensification of the stress. From this point onwards, the
behaviour of the specimen is similar to that of a bar containing
an internal flaw, the remainder of the metal is little affected by
further alternations of stress, but that in the immediate neigh-
bourhood of the crack or flaw is very greatly affected. An
important result obtained by Ewing and his co-workers, is that
fracture does not take place along the boundaries of the crystal
grains, but preferably through the grains, along the path of the
slip-bands.^

It has been seen that the production of a slip-band is due
to the gliding of crystal elements over one another along a
plane of translation, and that this action does not bring about
a change of crystal structure. As the stress is reversed, motion
along the same plane, but in the opposite sense, may take place.
If this were the only action, it would be possible for reversible
deformations to occur at each alternation, the displaced layers
gliding to and fro over one another without destroying the
crystalline structure of the metal. The fact that fatigue occurs
is itself a proof that the actual sequence of events is not so
simple. Further, one of the best-known phenomena in the
mechanical behaviour of metals is the increase of hardness
which results from the application of severe strains. A piece
of soft copper drawn into wire through a die, a silver button

' This statement is only made with reference to homogeneous materials.
In a substance containing layers of brittle intercrystalline material, such as
copper containing bismuth, the fracture naturally follows the crystal
boundaries.

352 METALLOGRAPHY

rolled out into a narrow strip, and a sheet of cartridge brass
forced into a cup shape by a hydraulic press, are familiar
examples of the hardening effect of severe mechanical work
on crystalline substances. Soft metals become hard and
elastic, and even brittle, under such treatment. It therefore
appears that the metal undergoes a molecular change when the
amount of deformation along gliding planes becomes excessive,
or when alternations of stress have led to repeated rubbing of
the gliding surfaces over one another.

Ewing and Humfrey observed that the slip-bands on the
polished surface of a sample of mild steel, after 40,000 reversals
of stress, showed a marked change in appearance. By careful
focussing it was found that a distinct ridge was produced along
the elevated edge of the slip-band, having the effect of a burr
produced by the friction. It is also known that a strained
plastic metal, such as iron, recovers its original properties to
some extent after being kept for some time, or very rapidly if
heated to a moderate temperature, such as 100°.^ This indi-
cates that the strained, hardened metal is in an unstable condi-
tion, and that its constituent molecules are under some degree
of restraint, from which they are prevented from escaping by
the high viscosity of the solid. Return to the stable condition
takes place spontaneously, although very slowly, and is enor-
mously facilitated by the reduction of internal viscosity due to
rise of temperature.

Experiments of this kind do not determine whether the
molecular change extends through the whole mass of metal, or
is confined to the immediate neighbourhood of the rubbing
surfaces. The latter is the more probable in view of the fact
that the hardening process is progressive, the hardness increas-
ing with the amount of work done on the solid. An ex-
planation covering all the facts is due to Beilby, based on
observations of the changes in the surface of crystals during
polishing. The results to be described in the next section
explain very satisfactorily the phenomena of hardening and
fatigue.

It remains to be said that fractures resulting from the

' J. Muir, Phil. Trans., 1899, 193a, I ; 1902, 198a, i.

PLASTIC DEFORMATION OF METALS 353

opening-up of slip-bands, consisting as they do of systems of
cleavage planes, commonly present a bright, crystalline appear
ance, which has led to the popular belief that metals subjected
to alternating stress become " crystalline." This is not so ; a
microscopical examination of the metal immediately behind
the fracture shows that the structure has not become coarser,
that there is no real increase in the size of the crystal grains,
but that the brilliant facets on the fractured surface are merely
cleavage planes. The same metal will exhibit entirely dif-
ferent fractures if broken under tension and under alternating
stress, the whole of the crystals being gradually deformed in
the former case, giving a fine fracture, whilst in the latter the
yielding is confined to a few cleavage planes. Fracture by
shock, causing separation to take place along the planes of
greatest weakness, without giving time for a readjustment of
stresses by the plastic yielding of other crystals, also produces
a coarsely facetted or " crystalline " surface.

Alternating stresses may be applied in several different
ways, in addition to that already described. In Sankey's appa-
ratus a flat bar, clamped at one end, is bent backwards and
forwards through a fixed angle ; in another form a bar, held
at both ends between rollers, is alternately thrust and pulled at
the middle point by a grip moved by an eccentric. Reversals
of direct stress have also been employed, the specimen being
placed alternately in tension and in compression.' Although
the methods of applying the stress vary, the process of yielding
is, in all cases, essentially that described above.

The Nature of Polish

It has been stated in Chapter VII. that there is a funda-
mental difference between the process of abrasion by a material
such as emery and that of polishing. The action of an abra-
sive material is one of cutting. Each particle cuts a fine
groove in the surface over which it rubs, the groove being
ploughed in a soft substance, or broken out in a series of
chips in a brittle substance. The form of the groove, which is
' T, E. Stanton, Proc. Inst. Mech. Eng., 1905, 897.
T.P.C. 2 -^

354 METALLOGRAPHY

made up of minute paraboloids, is approximately a parabolic
cylinder.^ The use of a somewhat finer abrasive material
effaces the coarser scratches by substituting for them a system
of finer grooves. This process may be carried very far, but
even the finest abrasive material only cuts grooves, although
these may be of microscopic fineness. Robert Hooke, who
examined the surface of a steel razor under his microscope,
and described the minute scratches which he observed, wrote,
in 1665^ — ■

" And indeed it seems impossible by Art to cut the surface
of any hard or brittle body smooth, since Puite, or even the
most curious Powder that can be made use of, to polish such a
body, must consist of little hard rough particles, and each of
them must cut its way, and, consequently leave some kind of
gutter or furrow behind it."

More recent research has shown, however, that the process
of polishing is of quite a different order. Polishing powders
consist of fine particles which may be, on account of their
softness, quite incapable of cutting grooves in the solid which
is being polished. Rayleigh ^ suspected that the polishing pro-
cess was really molecular in character. The union between
the powder and the polished surface is very intimate. If a
polishing cloth covered with rouge is allowed to become dry,
the rouge may attach itself so firmly to the metal surface that
it cannot be removed without destroying the smoothness of the
latter, and similar phenomena, indicating some degree of inter-
penetration between the two substances, are well known.

Beilby has shown * that the effect of polishing is to cause a
surface flow of the substance being polished. When an alloy
containing lead together with a harder metal is rubbed vigor-
ously on the polishing block, the lead is caused to flow, and
may cover the whole surface, completely masking the under-
lying structure. This effect is well known to beginners who

' F. Osmond and G. Cartaud, Rev. gin. Sciences, 1905, 16, 51.

' Micrographia, Observation II. This was also the opinion of
Herschel.

' Proc. Roy. Inst., 1901, 16, 563.

< G. T. Beilby, Proc. Roy. Soc, 1903, 72, 218, 226 ; Phil. Mag., 1904,
[vi.l 8, 258 ; y. Soc. Chem. Ind., 1903, 28, 1166.

A. Surface of bismuth, marked willi emcr}' scratches. X 200.

/>

B. Surface of bismuth, partly polished. X 200.

PLATE XII.

\To face page z<,'^.

PLASTIC DEFORMATION OF METALS -355

attempt to polish soft alloys. Flow takes place, although only
to a small depth, on the surface of even the hardest crystalline
substances, and Beilby has demonstrated the existence, on
polished surfaces, of a thin layer of flowed, structureless
material, strongly resembling in its behaviour a highly viscous
fluid. The " forced polish " produced by burnishing is due to
the formation of a deep layer of this flowed material. Its for-
mation is well shown on the surface of such a brittle metal as
antimony. A surface crossed by emery scratches is lightly
polished on a wet cloth block with alumina, and by examining
from time to time under the microscope, the gradual conver-
sion of a part of the metal into the viscous form may be
watched. If the direction of the polishing is kept constant and
transverse to the scratches, the flowed material is seen to form
an overhanging ledge or cornice at the edge of the grooves,
then extending to form a complete bridge. By forced polish-
ing it is easy to cover up even deep scratches in this way with
a flowed layer, presenting the appearance of a perfectly smooth,
polished surface. That the scratches are only covered, and
not removed, is readily seen on etching with a reagent, which
dissolves the film, and exposes the original grooved surface.
Beilby has shown by photographing at various stages that
grooves and pits on metals, and also on calcite, may be filled
up or bridged over in this way, and again exposed by the appli-
cation of an etching reagent. In the same way, an etched
pattern may be completely effaced by polishing, and a light
re-etching removes the film and again exposes the underlying
pattern. Plate XII., A, is a photo-micrograph of a surface of
bismuth crossed by numerous scratches produced by emery
paper. Plate XII., B, is a photo-micrograph of the same
specimen polished with alumina on a cloth. Most of the
scratches are completely covered by a film, some fine ones
appear broken, having been bridged in places, whilst one
broad scratcR is in process of being filled. It is from this
cause that etching a polished micro-section so often reveals
scratches which had apparently been removed during its
preparation and finishing.

The surface layer is harder than the mass of the solid and

356 METALLOGRAPHY

observations on calcite have shown ' that it is the same in all
directions, whilst the hardness of a crystalline substance is dif-
ferent in different directions. The orientating influence of a
crystal can, however, make itself felt under certain circum-
stances even through such a layer, as parallel growths of potas-
sium nitrate on calcite are found to occur even when the
surface of calcite used is not a fresh cleavage face, but one
prepared by polishing, and therefore covered with a viscous
film. The thickness of the film, in the case of calcite, is of
the order of lo'* mm.

The application of these facts to the hardening of metals by
work may now be seen. Wherever two crystalline surfaces rub

Fig. 104. — Formation of amorphous material in polished and strained
metal.

together, a viscous flowed layer is produced. The quantity of
the amorphous material produced in the gliding of one layer of
crystal elements over another may be insignificant, but each
repetition of the slipping causes it to increase. A piece of
metal subjected to alternating stresses exhibits slip-bands,
which are evidence that motion along gliding planes has taken
place in some of the crystal grains. In the course of the
to-and-fro motion of the crystalline layers, the rubbing surfaces
are converted more or less completely into the amorphous
material. As slipping does not take place readily along the
hardened surfaces, new gliding planes are opened up, and the
process continues until the supply of crystalline material is
considerably reduced, or until it is broken up into small
masses surrounded by and enclosed in amorphous hardened
metal. This condition is shown diagrammatically in Fig, 104,
the hardened material being represented in black. When this
' G. T. Beilby, Proc. Roy, Soc, 1909, 82a, 599.

PLASTIC DEFORMATION OF METALS 357

state of things is reached, alternating stresses very readily lead
to the formation of a crack, which therefore has its origin in
the breaking-down of the crystalline structure.

It is not possible, even by the most severe treatment,
such as drawing a ductile metal repeatedly through smaller
and smaller dies, to convert the whole of a mass of metal
into the amorphous modification. The distorted crystal grains,
even when drawn out into fine threads, retain a crystalline
core, merely enclosed in an amorphous shell.^ This sets a
limit to the amount of hardening which can be impressed
on a metal by work at the ordinary temperature. The hard
envelope probably prevents the enclosed crystalline portions
from yielding, so that fracture takes place when a further
stress is applied.

This theory explains the known facts of hardening and
polishing with remarkable completeness. The amorphous
modiScation has much analogy with a vitreous substance, such
as a glass or undercooled liquid. It must be regarded as
unstable at all temperatures, and only prevented from return-
ing to the stable, crystalline condition by its internal viscosity.
Hence the tendency, already mentioned, to spontaneous
recovery from overstrain, especially when the temperature is
raised. The complete crystallization at higher temperatures
will be considered immediately.

The suggestion has been advanced recently ' that the pro-
cess of flow is one of actual melting. Although an increase of
pressure generally raises the melting-point of a metal, pressure
acting locally in such a way that the liquid formed is free to
escape has the effect of lowering the melting-point.' Calcula-
tion shows that lead would melt at 27° under a pressure of
1760 atmospheres, and copper at the same temperature under
24,000 atmospheres. The order of the pressures required to
melt the metals, as calculated from their latent heat of fusion
and their density, corresponds perfectly with the observed
order of plasticity. It is, however, difficult to imagine that

' G. T. Beilby, Proc. Roy. Soc, 1905, 76a, 462 ; 1907, 79a, 463.
' J. Johnston, /. ^mer. Cliem. Soc, 1912, 34, 788.
3 J. H. Poynting, Phil. Mag., 1881, [v.] 12, 32.

358 METALLOGRAPHY

such enormous pressures can be attained locally, and yet be
confined to the crystals.

On the other hand, it has been maintained that the harden-
ing process is one of the repeated subdivision of the crystals
along cleavage planes, and that an amorphous material is not
produced at any stage.^ This view has been developed in
great detail, but appears to take insufficient account of the
evidence from microscopic structure, and from the observation
of polished surfaces. A molecular theory of the alteration
produced in crystalline substances by fatigue, depending on
the assumption that the molecules or crystal-elements possess
polarity of some kind, has been elaborated by Ewing.^

Thin Films

The formation of the amorphous modification, and its
recrystallization under the influence of a rise of temperature, are
very conveniently studied in thin metallic films, such as gold-
leaf, which are prepared by beating. It was observed by
Faraday as far back as 1857' that a goldJeaf supported on
glass becomes transparent, and loses its reflecting power, when
heated to a very moderate temperature. Thin beaten gold
leaf is truly transparent, transmitting green light. Treatment
of such a film with a dilute solution of potassium cyanide
causes the removal of the amorphous modification, leaving the
crystalline residue in a spongy form. The effect of heating
the foil supported on glass is to give sufficient mobility to
the film to allow it to behave as if fluid, so that its particles
gather themselves into aggregates apparently under the influence
of surface tension. Hence the increased transparency, and the
transmission of white instead, of green light, is due to this
aggregation into globules, between, and not through, which the

' O. Faust and G. Taaimann, Zeitsch. physikal. Chem., 191 1, 76, 118 ;
G. Tammann, Zeitsch. Elektrochem., 1912, 18, 584.

°- Brit. Assoc. Rep., York, 1906, Presidential Address to the Engineer-
ing Section.

' Phil. Trans., 1857, 147, 145 ; Exp. Researches in Chemistry and
Physics, 1859, 391.

PLASTIC DEFORMATION OF METALS 359

light passes. This is confirmed both by direct microscopical
observation and by measurements of the electrical resistance,
which increases from a small value (0-2 to 50 ohms) for the
original foil, up to thousands of megohms for a specimen ren-
dered transparent by heating at 300°.^ At so high a temperature
as this, the gold is rendered crystalline, but the discontinuity
produced by aggregation while still partly amorphous is the
cause of the increased resistance. The green colour and
brilliant metallic lustre must be attributed to the flowed layer.
They can be restored to the gold leaf crystallized by heat by
burnishing, a process which re-forms the flowed layer. Silver
resembles gold in these respects, a fact also observed by
Faraday.

The properties which distinguish metals and alloys hardened
by mechanical work from the same substances in a state of
equilibrium are due, on this view, to the presence of an
amorphous modification in the former. The electrical resist-
ance is raised by straining, and the original value is regained
after annealing at a temperature sufficiently high to cause
recrystallization. The elasticity which enables a soft copper
or silver wire, after drawing, to act as a spring, is an even more
characteristic property of strained metals than their hardness.
The electric potential is changed, a part of the energy which
has been absorbed during the working reappearing when the
metal is placed in contact with an electrolyte in which it can
dissolve. Hence a worked metal, connected with a piece of
the same metal in an annealed or fully crystalline condition, to
form a voltaic cell, becomes the anode,^ and develops an
increased quantity of heat in the process of solution.' This
existence of a difference of electrolytic potential between
strained and unstrained metals is of considerable importance
in dealing with the probability of corrosion in structures in
which worked and unworked metals are in contact.

' Beilby, loc. cit. The changes in gold and silver films have also been
studied by T. Turner, Proc. Roy. Soc, 1908, 8U, 301.

2 C. E. Fawsitt, Proc. Roy. Soc. Edin., 1906, 25, 2 ; J. Soc. Chein.
Ind., 1906, 25, 1 133.

2 M. Berthelot, Compt. rend., 1901, 132, 234; C. Barus, Bull. U.S.
Geol. Survey, Bull. 94.

36o METALLOGRAPHY

A property which may be very conveniently employed in
the study of the hardened modification is the thermo-electric
force developed when the hardened and annealed specimens
are placed in contact.^ Using silver wires, Beilby has observed
an E.M.F. as high as 0-17 microvolt per 1° of difference
between the temperatures of the hot and cold junctions. By
raising the temperature and measuring the E.M.F. from time to
lime, the point at which annealing sets in may be observed.
When the annealing process is complete the two wires become
identical, and the thermo-electric force falls to zero. With
most metals, the annealing change sets in somewhat suddenly,
so that the E.M.F. curve exhibits an abrupt discontinuity; but
this point cannot be regarded as a true transition temperature,
as one of the modifications has no range of stability, but is
unstable at all temperatures. The break in the curve represents
the temperature at which the molecular mobility becomes
sufficient to permit crystallization to take place.

The recrystallization of gold begins at 250°, that of copper
at 2 00°. 2 The temperature is higher for alloys, being near
300° for brasses containing 10 per cent, or 33 per cent. Za?
After heating at temperatures above these, the process of
recrystallization may be watched microscopically.

The behaviour of some metals and alloys when subjected
to stress at high temperatures has, however, led to the sug-
gestion ' that the amorphous phase disappears only at a higher
temperature, which has been determined to be 650° for copper,
395° for aluminium, and 710° for an alloy of 80 per cent, of
copper and 20 per cent, of nickel. It is not easy to reconcile
such a supposition as that just described with other facts, and
the experimental results are perhaps susceptible of a different
interpretation.

' M. Maclean, Proc. Roy. Sac, 1899, 6*, 322 ; 1900, 66, 165.
' Beilby, /oc. cit.

' A. Portevin, jRezi. de Metallurgie, 1909, 6, 814; see also A. Le
Chatelier, Ginie civil, 1891, 19, 59.

' G. D. Bengough, J. Inst. Metals, I912, 7, 123.

CHAPTER XVII

THE METALLOGRAPHY OF IRON AND STEEL

The technical importance of the metallography of iron and

steel, and the extraordinary complexity of the relations

exhibited by these metals, justify the separate treatment of

this group, the literature of which exceeds in volume that of

all the other departments of metallography taken together.

The equilibrium of iron and carbon will therefore be considered

in some detail, a briefer account being subsequently given of

the modifications brought about by the introduction of other

elements into the system. An outline of the constitution of

metallic meteorites will conclude the chapter.

Molten iron dissolves carbon very readily up to a certain

limit. A saturated solution at 1400° contains about 6 per

cent. C, and although much more may be dissolved by raising

the temperature, the excess is deposited on cooling in the

form of graphite, which separates very readily on account of

its lightness, so that it is difficult to obtain solid alloys

containing more than 6 per cent. C, and even this value can

only be obtained under certain conditions. The very light,

crystalline graphite which collects on the surface of iron rich

in carbon is well known in iron works under the name of

" kish " (Germ. Garschauni).

That the melting-point of iron is depressed by the addition

of carbon, is one of the most familiar facts of the iron industry.

An alloy containing 4 per cent. C melts near 1100°, or 400°

below the melting-point of pure iron. The first freezing-point

diagram was constructed by Roberts-Austen in 1899,* and the

' 5th Report, Alloys Research Committee, Proc. Inst. Mech. Eng.,

1899, 3S-

361

362 METALLOGRAPHY

data which it contained were utilized in the following year by
Roozeboom ^ in the construction of a complete diagram of
the phase equilibrium. To give an account of the modifica-
tions which this diagram has undergone at the hands of later
investigators, and of the controversies to which the proposed
modifications have given rise, would occupy more space than
can be given here, and reference must merely be made to
some of the most recent reviews of the subject.^ The diagram
represented in Fig. 105 embodies the conclusions which are
most widely accepted at the present time, certain of them
being avowedly provisional. A somewhat revolutionary
diagram, recently proposed, will be indicated later.

The chemical study of iron and steel, which preceded the
metallographic study, had established the fact that carbon
may exist in steel or in cast-iron in three different forms,
namely graphite, " carbide carbon," and " hardening carbon."
Of these, the first is identical with natural graphite, the second
corresponds with a carbide which is known to have the compo-
sition FejC, whilst the third form of carbon is present in a
state of solid solution. The diagram has to account for these
different conditions.

Beginning at the left-hand side of Fig. 105, tlxe freezing-
point of iron is taken as 1505°.* Iron is now generally
considered, in accordance with Osmond's hypothesis, to
exist in three allotropic modifications. Although all three
forms crystallize in the regular system, and crystallographic
differences between them are doubtful, yet the undoubted dis-
continuity of properties at certain well-defined temperatures,
marked by energy and volume changes, prove the fact of
physical transformations which are conveniently termed
allotropic. Direct evidence of the allotropy has been obtained

" Zeilsch. physikal. Chem., 1900, 34, 437 ; J. Iron Steel Inst., 1900,
ii. 311.

' P. Goerens, Erstarrungs- und Erkiiltungsvorgmige bei EisenkohUn-
stofflegiei-iingm, Halle, 1907; H. M. Howe, Metallurgie, 1909, 6, 65, 105 ;
F. Wiist, ibid., 512.

" H. C. H. Carpenter, J. Iron Steel Inst., 1908, iii. 290 ; in good
agreement with B. Saklatwalla, ibid., ii. 92, and H. Harkort, Metallurgie,
1907, 4, 646.

METALLOGRAPHY OF IRON AND STEEL 363

by straining a strip of iron, heated by the passage of a current
in vacuo. Within the ranges of temperature corresponding
respectively with the stability of the three modifications, different
systems of slip-bands are obtained, indicating distinctly different
mechanical properties.' The y-iron, which forms at the freezing-

a • Ferrite + Cementitt

Fig. 105. — Alloys of iron and carbon.

e %C

point, has a considerable solvent power for carbon, and is the
only modification capable of forming concentrated solid
solutions. At about 900° it passes into the /3 form, the change
being accompanied by a development of heat, and by a
considerable expansion of volume. This critical point is
denoted by Arj (A standing for arret, r for refroidissement. The

' W. Rosenhain and J. C. W. Humftey, Froc. Roy. Soc, 1910, 83a, 2cjo.

364 METALLOGRAPHY

corresponding change on heating is denoted by Acs, c standing
for c/iaufage; the two points coincide under conditions of,
equilibrium^. At 760°, /S-iron changes into the a modification,
with development of heat, but with very little alteration in
volume. This point is termed Arg, and is less sharply marked
than Arj.

It is not certain whether ;8-iron has the property of
retaining carbon in solid solution.

y-Irgn is not itself hard, but is capable of forming sdid
solutions with carbon in the form of carbide, which become
hard under certain conditions of cooling. In the absence of
carbon, the transformation into /8- and a-iron takes place with
such facility that even rapid quenching is insufficient to retain
it in the y form.

Returning to the upper part of the diagram, the freezing-
point of iron is depressed by carbon, the curve having the
form indicated. A eutectic point is reached at 4*3 per cent. C
and 1125°. The solid phase which separates along this
descending branch of the curve is not pure iron, but a
solid solution of carbon in y-iron, which, when saturated
at 1 1 25", contains about i'8 per cent. C, indicated by the
point E.

The determination of the exact form of the solidus AE is
difficult by purely thermal means, as the end of solidification
is not very distinctly marked on the cooling curves, but it has
been accomplished by the application of Heycock and Neville's
method of quenching. Several specimens of the same composi-
tion are quenched from temperatures near to the supposed
position of the solidus. Microscopical examination then
shows which of these specimens contained liquid material
at the moment of quenching. Another, quenched from a
slightly lower temperature, is found to have been completely
solid, and by a repetition of such experiments the point at
which the cooling curve of each alloy intersected the solidus
is determined.^

To the solid solution separating along the branch AB
is given the name of austenite. Its physical properties, when
' X. Gutowsky, Metallurgie, 1909,6, 731, 737.

METALLOGRAPHY OF IRON AND STEEL '365

preserved by rapid quenching, will be described later. Its
limit of saturation is reached at i"8 per cent. C. Solutions
containing a larger quantity of carbon deposit austenite until
this concentration is reached, and the remainder of the liquid
solidifies as saturated austenite together with a eutectic. The
nature of the eutectic depends on the conditions of experiment.
We will first assume that precautions are taken to exclude
silicon, and that the rate of cooling during solidification is
not unduly slow. In this case, the solid phase in equilibrium
with austenite at 1125" is cementite FcsC, which does not form
solid solutions. Free cementite crystallizes from liquid alloys
containing more than 4^3 per cent. C, as indicated by the
curve BC. At the eutectic point C, the two phases, cementite
and saturated austenite, crystallize simultaneously. This eutectic
structure is observed in white cast-iron.

On cooling saturated austenite below 1125°, its concentra-
tion is reduced by the separation of crystals of cementite, the
solubility curve having approximately the form ES.' At
690° the limit of saturation of the solid solution is only o'g
per cent. C.

The transformation of 7- into /3-iron also occurs where the
■y-iron is present in the form of a solid solution. In accordance
with the general behaviour of solid solutions, the temperature
of the transformation is lowered by the presence of carbon.
Consequently, in a steel containing a small quantity of carbon,
the transformation Arj does not take place at 900°, but at a
lower temperature. The curve GO represents this change.
It is uncertain whether j8-iron retains carbon in solid solution
or not. We may assume provisionally that it does not, and
that austenite consequently deposits pure^, carbon-free iron
when the curve GO is crossed. The temperature of the
second transformation Arg, by which j8-iron changes into a,
is then unaffected by the presence of carbon, as it is confined
to the separated pure metal. The line MO is therefore drawn
as a horizontal line. Should /3-iron prove to have a small
solvent power for carbon, this line must be more or less

' The form of the curve ES has been taken from the results of N. J.
Wark, Metallurgie, 191 1, 8, 704.

366 METALLOGRAPHY

broken, but the diflerence is not material. From the point O
onwards, the transformation Arg vanishes, or coincides with
Ars, as a-iron separates directly from the solid solution of
carbide in y-iron along the curve OS, without an intermediate
passage through the j8 stage. The transformation along OS is
often denoted by Arj-j. On reaching the composition o'g per
cent. C, this line intersects the curve of separation of carbide,
and the point of intersection is a. entectoid -point (p. 59). It
represents the breaking-up of the residual solid solution, con-
taining o"9 per cent. C, into a complex of ferrite (iron
containing very little, if any, carbon), and the carbide,
cementite. This intimate eutectoid mixture is known as
pearlite.^

The behaviour of several alloys, containing different
percentages of carbon, may now be examined, when they
are cooled from a temperature immediately below the solidus.
A steel containing o'j per cent. C will have the following
arrest points —

Ars 820°, Arj 760°, Arj 690°.

With an increase of the carbon to 07 per cent., the points Arj
and Ar2 coincide, and we have two arrests only —

Ar3^73o°, Ari69o°.

The eutectoid alloy, with o"9 per cent. C, has only a single
arrest-point, Arj, at which the change of y- into a-iron, and the
resolution of the solid solution into ferrite and cementite, take
place simultaneously. As Ari is the recalescence point, already
referred to (p. 209) the magnitude of the recalescence effect is
a maximum for steel of this composition. Passing to the alloy
containing i'5 per cent, of carbon, we again meet with two
arrest points, the first, Arj^, corresponding with the separation
of cementite from the solid solution, and the second, Ar^, with
the resolution of the eutectoid mixture. Whatever be the

' The form of the curve GOS has been determined by means of
systematic quenching experiments by P. Goerens and H. Meyer,
Melallurgie, 1910, 7, 307.

METALLOGRAPHY OF IRON AND STEEL 367

composition of the initial solid solution, it deposits a part
of its iron whenever the point representing its composition
intersects the curve GOS, or a part of its cementite if it
intersects the curve SE. The solid solution remaining when
the temperature has fallen to 690° has in every case the
composition represented by the point S, and is transformed
into pearlite.

The diagram just described does not represent the whole
of the facts, although it affords a clue to the behaviour of the
alloys comprised within its range. The solid solution, referred
to provisionally as austenite, undergoes partial resolution so
rapidly when passing through the critical range of tempera-
ture, that even the most rapid quenching fails to retain the
solid solution formed on freezing, in a metastable state. In
ordinary quenching experiments, therefore, one obtains mix-
tures of austenite with ferrite and cementite in a very finely
divided form. Several of the stages in the resolution of the
solid solution lead to the production of such characteristic
microscopic structures that distinct metallographic names have
been given to them; but as none of these represent con-
ditions of stable equilibrium, they are not included in the
diagram.

The stages generally recognized are —

Austenite — martensite — troosite — [osmondite] — sorbite —
pearlite.

They will be described together with the other micrographic
constituents of the iron-carbon alloys.

The facts that slowly cooled cast-iron contains practically
the whole of its carbon in the form of graphite, and that
cementite, or iron rich in cementite, deposits carbon when
heated, are well known, but do not receive recognition in
the equilibrium just described. An important series of
experiments led to the conclusion ^ that the final equilibrium
of the system, only reached after a very prolonged annealing,
was one between the two solid phases ferrite and graphite.
The suggestion was then made that two systems, a stable and

' G. Charpy and L. Grenet, Bull. Soc. cF Encouragement, 1902, 399. '

368 METALLOGRAPHY

a metastable, should be represented in the diagram,' and this
representation has been adopted by the majority of metallur-
gists, although open to considerable objection.

In the metastable system, the phases separating from the
liquid alloy are austenite and cementite, as described above.
In the stable system, on the other hand, they are austenite and
graphite. The eutectic point lies somewhat above that of the
metastable system, but not far from the same percentage of
carbon. The curve AF, representing the crystallization of
austenite, is the same in both systems. The graphite curve,
FG, is placed somewhat above the cementite curve, CB, and
the carbide system is thus represented as being produced by
the undercooling of the graphitic system, AF being prolonged
to C. The supposed curves of the stable system are here
drawn as dotted lines.

According to Goerens, solidification never takes place
according to the scheme indicated by the dotted lines."
This investigator concludes from his experiments that under-
cooling always takes place at least as far as the eutectic
point, and that the liberation of graphite is always due to the
dissociation of the cementite first formed. Benedicks and
others have found a solubility for graphite in y-iron correspond-
ing with the dotted curve HP.' This conclusion is reached
principally as the result of measurements of the extent to
which iron can dissolve graphite at different temperatures, and
is open to the objection that in such cases an intermediate
formation of cemenite may take place. It is certainly a
suspicious circumstance that the curves AF, FG, HF, and
HP coincide almost exactly with AC, CB, EC, and ES,
respectively, and this fact alone leads to some hesitation
in accepting the proposed scheme of stable and metastable
equilibria.'' The hypothesis of an equilibrium between graphite,

' G. Charpy, Compt. rend., 1905, 141, 948 ; E. Heyn, Zeiisch. EUkro-
chem., 1904, 10, 491.

■ Metallurgie, 1907, 4, 137.

- G. Charpy, Rev. de Metallurgie, 1908, 6, 77 ; R. Ruer and N. Iljin,
Metallurgie, 1911, 8, 97.

' See p. 378.

A. Soft iron, made by direct process, x 200.

'^'.v-^:

E, Soft steel, x 200.
PLATE XIII.

\To fate pa^e 369.

METALLOGRAPHY OF IRON AND STEEL 369

iron and carbide in solution, Fes C ^ 3 Fe + C, has often
been made, and may be used with some success to explain the
phenomena.^

We may now proceed to describe the microscopic appear-
ance of the constituents enumerated in the diagram, and of
the well-defined stages in the passage of steels from metastable
to stable equilibrium.

Ferrite. — The name ferrite is given to a-iron. It forms the
sole constituent of pure soft iron, and then occurs in poly-
hedral grains, the outlines of which are developed on etching,
owing to minute differences of potential at the junction of
neighbouring grains. Etching with picric acid produces a
marked roughening of the surface, whilst nitric acid colours
the grains, owing to the formation of oxide films. Different
grains, owing to variations of orientation, become coloured
unequally. I*" the iron is not quite pure, the impurities are
commonly found at the edges, and especially in the angles,
between adjacent grains. A specimen of nearly carbonless
iron (a Japanese sword-guard, made by the direct process) is
photographed in Plate XIII., A. Small quantities of slag are.
seen in the form of inter-crystaUine enclosures, but with these
exceptions, the specimen consists of pure ferrite.

Deep etching with acids, or with cupric ammonium chloride,
reveals a cubic structure in ferrite. Attempts have been made
to determine the crystallographic characters of y- and )8-iron by
reducing crystals of ferrous chloride in hydrogen at tempera-
tures above their respective transformation points.'' As the
result of such experiments, it has been found that y-iron
crystallizes in combinations of the cube and octahedron,
whilst no difference could be detected between /8- and a-iron,
both of which crystallize in cubes. It must, however, be
admitted that the crystallographic relations of the three modi-
cations of iron are still obscure.

a-Iron containing silicon or other elements in a state
of solid solution is indistinguishable from pure iron, and is

' See, especially, A. Smits, Zeitsch. Elektrochem., 1912, 18, S' 5
O. Ruff, Metallurgie, 191 1, 8, 456; H. M. Howe, Trans. Amer. Inst.
Min. £ng., 1912, 1 181.

« F. Osmond and G. Cart^ud, Ann. dies Alif^es^ 1900, [ix.] 18, 113.

370 METALLOGRAPHY

therefore also considered as ferrite. Some solid solutions of this
kind develop the cubical structure with remarkable perfection
when etched deeply, and the best photo-micrographs of this
sort have been obtained from iron containing silicon. It has
been proposed that the micrographic constituent of alloys con-
taining phosphorus, silicon, etc., should be called phospho-
ferrite, silico-ferrite, etc.

The mechanical enclosures in ferrite (silicate slag and
manganese sulphide) have been referred to in Chapter
IX., and globules of the sulphide are seen in Plate
XIII., B.

Graphite. — Graphite occurs in grey cast-iron in the form of
large, thin plates, crystallizing in the hexagonal system.
These plates are very commonly curved, the soft, plastic
character of the graphite allowing it to yield to the pressures
produced in the growth of the neighbouring ferrite crystals.
The presence of both primary graphite, separating along the
curve FG, and of eutectic graphite, formed at the point F,
is rarely observed, owing, on the one hand, to the reluctance of
the iron-carbon alloys to enter the state of stable equilibrium,
and on the other to the low specific gravity of graphite, which
causes it to fioat up to the surface of the solidifying mass.
A good photo-micrograph, showing both the primary and
eutectic graphite, was obtained by Goerens ' from a grey iron
containing 7 per cent, of graphite, prepared in the electric
furnace.

Graphite is at once visible in the polished section, without
etching. In specimens as ordinarily polished, without special
precautions, the graphite is removed during the grinding
process, and its place is represented by fine cavities. This is
the usual result obtained in preparing sections of grey cast-iron.

Temper carbon is a finely divided form of graphite, pro-
duced in the decomposition of cementite, or of a solid solution
of carbon in iron ; it is therefore produced in the annealing of
white cast-iron. It is commonly visible in the etched sections
as minute aggregates, or " nests " surrounded by white areas of
ferrite, due to the withdrawal of carbon from the surrounding

1 Melallurgie, I907, 4, 137.

METALLOGRAPHY OF IRON AND STEEL 371

alloy by segregation. Temper-carbon has been proved to be
chemically identical with graphite.^

Cementite. — The carbide, cementite, FejC, is the only com-
pound of carbon and iron which has yet been recognized by
microscopical means. It is present in meteorites as cohenite.^
A second carbide, FejC, has been isolated by chemical means
(see p. 377), and is present as chalypite in certain meteorites,^
but we are at present unable to distinguish the two compounds
in micro-sections. Cementite crystallizes in large plates,
apparently belonging to the hexagonal system,^ although
cohenite is stated to be cubic." When seen in a micro-section
the plates, being cut transversely, have the appearance of
needles. In a white iron containing more than the eutectic
proportion of carbide, the crystals are very well formed and
regular, in annealed steels containing more than the eutectoid
proportion they form more or less irregular bands arranged as
cell walls enclosing areas of pearlite.

Cementite is not attacked by acid etching reagents, and
remains brilliantly white when ferrite and pearlite have been
darkened by etching. On the other hand, cementite is
blackened by a boihng solution of sodium picrate in an excess
of sodium hydroxidfe, and this reagent is the most characteristic
for distinguishing cementite from a solid solution. The hard-
ness of cementite (6 on Mohs' scale) causes it to stand out in
relief when the specimen is polished on a soft bed, and draw-
ing a needle over the surface may also be employed as a
means of recognition.

Pearlite. — The eutectoid mixture of ferrite and cementite
is known as pearlite. In its most perfectly developed state,
as seen in a slowly cooled steel, the two phases are arranged
in parallel, slightly curved or wavy, lamellae. If the annealing
be prolonged, this arrangement, which is mechanically unstable,
is changed into one in which the cementite forms granular

' G. Charpy, Compt. rend., 1907, 145, 1173.
' E. Weinschenk, Ann. Museum. VVien, 1889, 4, 94.
» C. U. Shepard, Amcr. J. Sci., 1867, [ii.] 43, 28.
* Stated by Groth, Chein. Krystallographie, 1906, i., to be pseudo.
hexagonal,

» L. J, Spencer, Min. Mag., 1902, 13, 296,

372 METALLOGRAPHY

masses or oval globules, surrounded by areas of ferrite. This
granular pearlite may be of all degrees of coarseness, the
final stage being one in which the whole of the cementite is
collected into a few isolated masses. Plate XL, B, shows
pearlite which is granular in parts of the field, but chiefly
lamellar. An interesting intermediate form has been observed,
the cementite presenting itself as lamellae, which appear con-
tinuous under a low power, but under higher magnification
resolve themselves into series of globules, resembling strings
of beads.' This " beady " pearlite represents the first stage in
the transition from lamellar to granular pearlite.

Pearlite is more readily etched by acid reagents than
ferrite, so that it appears as dark patches when an annealed
steel is etched and examined under a low magnification.
When more highly magnified, it is seen that only the ferrite
lamellje are darkened. Polishing in relief, or polishing by
Osmond's polish-attack method, also brings out the cementite
lamellae on a dark ground. In Plate XIII., B, representing
a low-carbon steel, the pearlite appears as black patches,
being unresolved at the low magnification adopted. The
laminated structure is the cause of the pearly lustre of the
etched or relief-polished surface which has gained for pearlite
its name.

The cementite of pearlite, as ordinarily obtained, appears
to be identical with structurally free cementite, chemical
analyses of the two being identical.^ The question whether
the ferrite constituent is identical with structurally free ferrite
is not so easily solved. The fact that ferrite, when forming a
constituent of pearlite, is much more rapidly etched than when
free, suggests a chemical difference, although it is possible to
explain it as due to the local electrolytic influence of the
cemenite. Benedicks concludes from the density, magnetic
properties, etc., of steels, as well as from the micro-structure,
that a solid solution of carbon in /3-iron, containing up to o"27
per cent. C, is present in pearlite, and to this constituent he
gives the name of ferronite. The existence of ferronite as a

' C. Benedicks, Metallurgie, 1909, 6, 567.

' a! Ledebur, .S/a/j/ «. Eisen, 1887, 8, 742; 1891, 11, 294.

METALLOGRAPHY OF IRON AND STEEL 373

distinct phase has not received general acceptance from
metallographists.

01 2 3*5

Fig. io6. — Micrographic constituents of steels.

6%C

Atisfenite. — The term austenite has been variously employed
in the past, but its use is now confined to the solid solution
formed in the solidification of iron-carbon alloys. It therefore
consists essentially of a solid solution of iron carbide in
7-iron, but other elements may also be present in a dissolved
state without altering its nature. It has not been found
possible to prepare pure austenite by quenching a pure iron-
carbon alloy, a certain amount of transformation taking place
while passing through the critical range, however rapidly the
quenching is carried out. The stability of austenite is, how-
ever, greatly increased by the presence of manganese, and the
pure constituent has been obtained' by quenching a steel
containing o'93 per cent. C and i'67 per cent. Mn from 1050°
in ice- water. Steels containing 13 per cent. Mn or 25 per
cent. Ni (manganese or nickel steels) consist of pure austenite.

Homogeneous austenite forms polygonal grains, frequently
twinned, and resembling in general appearance an annealed
brass. When growing freely, its crystals are regular octahedra.
It is very soft, being little harder than ferrite, and is non-
magnetic. As it occurs most frequently in the form of mixtures

' E. Maurer. Metallurgie, 1909, 6, 33.

374 METALLOGRAPHY

with martensite, its metallographic behaviour will be described
under the next heading.

The austenitic structure of steels at a high temperature
may be revealed by etching with gaseous hydrogen chloride at
1120°, even in the absence of manganese.^

Martensite. — ^This is the principal constituent of iron-
carbon alloys hardened by rapid quenching. It is the essential
constituent of hard steels, and is the cause of their hardness.
It represents the first stage in the resolution of austenite
during cooling into a-iron and cementite. A quenched, but
untempered, tool-steel shows the structure well. When etched
with picric acid, it is resolved into systems of very straight,
parallel lines, forming three systems intersecting at angles of
60°. These lines, often described as needles, really represent
cleavages of octahedra. Long supposed to be homogeneous,
and to represent the original solid solution stable above the
transformation point, the recent researches on austenite have
proved that martensite is a product of partial resolution, and
that the etching reveals a heterogeneity of structure. Mar-
tensite contains a-iron, and is therefore magnetic, but the
constrained molecular condition gives it a high coercive force.
It is the essential constituent of permanent magnets.

A steel containing f6 per cent. C, quenched from 800"
in ice-water, consists of cementite, in a ground-mass of pure
martensite, whilst the ground-mass of the same steel, quenched
from 1100°, consists of a mixture of austenite and marten-
site. In such mixtures, the martensite forms characteristic,
angular crystals, designated " fers de lance " by Osmond. The
behaviour of these crystals on etching is variable. Sometimes
they appear black on the white background of austenite, at
other times the austenite is etched more rapidly than the
martensite. Kurbatoff s complex reagents produce better con-
trasts than the simple acids commonly used for etching. A
needle may also be used to distinguish austenite from marten-
site, the greater hardness of the latter being very marked.
The different behaviour on etching is frequently due to the

' Baykoff, Rev. de Metallurgie, 1909, 6, 829 ; N. J. Wark, Metallurgie,
1911, 8, 731.

METALLOGRAPHY OP IRON AND SXEEL 375

partial conversion of one or other of the constituents into
troostite.

Austenite is partly converted into martensite by cooling in
liquid air,' a polished surface after such treatment being
marked with " fers de lance " in relief, or the same change
may be brought about, to a limited extent, by cold-working,
that is, by mechanical deformation at the ordinary temperature.

Troostite. — This constituent, frequently present in hardened
steels, especially after partial tempering, is characterized by
the deep brown or black surface tint produced by etching,
especially with reagents containing nitro-compounds. It
commonly presents itself as nodules, or as a border invading
crystals of austenite or martensite. Various suggestions have
been made to explain the nature of troosite. The hypothesis
that it is ;8-iron, free from carbon,'' may be at once rejected,
whilst the view that it consists of a solution of elementary
carbon in iron,' austenite being a solution of carbide, is also
without, evidence in its favour. The physical and chemical
properties of troosite all indicate that it is a mixture of
cementite and ferrite, only differing from pearlite in its
extremely fine state of division. Benedicks describes it as a
colloidal solution of carbide in iron.* It was described by
Arnold as " emulsified carbide," ^ a term which also ex-
presses the fine state of subdivision. It may be obtained in
an almost pure condition by quenching steel containing o'g per
cent, of carbon from 725° with suitable rapidity. The co-
alescence of the carbide particles to form pearlite takes place
rather sharply at 400°. The fine state of division is sufficient
to account for the readiness with which carbon is liberated
from it by the action of etching reagents. The darkening of
austenite or martensite in tempered specimens is due to partial
conversion into troosite.

' F. Osmond, Etude des AUiages, 296 ; E. Maurer, loc. cit,
' H. C. Boynton, J. Iron Steel Inst., 1904, i. 262.
= W. A. Kurbatoff, Rev. de Mitallwgie, 1905, 2, 169. See also
F. Rogers, y. Iron Steel Inst., 1905, i. 484-

• Ibid., 1905, ii. 352 ; 1908, ii. 217 ; Kolloid Zeitsch., 1910, 7, 290.
' J. O. Arnold, Proc. hist. Civ. Eng., 1^95, 188, 127.

376 MkfALLOGRAPHV

Osmondite. — Heyn and Bauer ^ have given the name osmon-
dite to a stage intermediate between troostite and sorbite, and
forming the boundary between these two structures. They
were led to adopt a distinct name by their observation that
the passage of martensite into pearUte on heating takes place in
such a way that the transformation has a maximum value at
400°. Steel heated to this temperature has a maximum rate of
solution in sulphuric acid. The quantity of carbon which is
liberated in the elementary state on dissolving in 10 per cent, sul-
phuric acid (as distinguished from that which is liberated as car-
bide) increases from martensite through troostite to osmondite,
and then diminishes on passing from osmondite through sorbite
to pearlite. No characteristic microscopic structure is, however,
observed, and it therefore seems superfluous to distinguish the
boundary between troostite and sorbite by a new name.

Sorbite. — Sorbite differs only in its minutely granular
structure from pearlite. It becomes coloured like pearlite by
etching reagents, but higher magnification reveals nothing but
a finely granular structure. Structural steel contains more
sorbite than laminated pearlite, the former being accompanied
by better mechanical properties. In a white iron, such as that
shown in Plate XIV., A and B, the original crystallites of
austenite have been converted during cooling into sorbite,
which appears dark and structureless in the photograph.

The Technical Varieties of Steel and Cast Iron

The structure of steels in an annealed condition may be
determined from Fig. 106. The area of pearlite in a photo-
micrograph increases rapidly with the proportion of carbon
up to o'9 per cent., and then diminishes more slowly. The
pearlite in very low-carbon steels forms small areas between
the ferrite grains, but as its proportion increases a different
structure appears, and the ferrite forms a network, enclosing
the areas of pearlite. The size of mesh of this network is an
indication of the size of the grains of austenite existing at a
high temperature before the transformation took place.^ In

• y. Iron Steel Inst., 1909, i., 109.

' H. M. Howe, Jnternat. Zeitsch. Metallogi-aphie, 1912, 2, 13.

A. White pig iron, x 86.

,i4^^:^^.

/-§"

^*-V»>fX;;

?»..,

,^^" :>...>•/>■. /

B. Eutectic of white pig iron, x 200.

PLATE XIV.

[7»/(z«/fffif 376.

METALLOGRAPHY OF IRON AND STEEL 377

hyper-eutectoid steels, the cemeiitite forms a similar network,
the structure in both cases being due to the deposition of the
constituent in excess at the borders of the grains during
cooling. Much information as to the rate of cooling may be
derived from a study of the thickness of the borders in any
given case.

The structure of hardened steels is too complex to be dis-
cussed here in any detail. Reference may be made to a useful
review of the subject from the physico-chemical point of view."
The view there maintained, however, that the difference
between the constituents austenite and martensite is merely
due to twinning under the influence of pressure, is not generally
accepted, and is open to many objections. The conditions
under which different structures occur have been briefly
indicated under the headings of the respective constituents.

White cast-iron owes the principal features of its structure
to the presence of the austenite-cementite eutectic, the appear-
ance of which is shown in Plate XIV., B. If the proportion
of carbon is less than 4'3 per cent., the eutectic is accom-
panied by crystal skeletons of primary austenite, as shown in
Plate XIV., A. During cooling, these austenite masses under-
go resolution, and generally appear as dark masses of sorbite
after etching, but their outline remains that of the primary
austenite. Needles of secondary cementite, thrown out of
solid solution along the curve ES (Fig. 105) are often to be
seen in the dark mass of sorbite.

On the other hand, white iron containing more than
4'3 per cent, of carbon shows large plates of primary cementite
in addition to the eutectic. An iron containing exactly the
eutectic proportion of carbon frequently exhibits a cleavage
suggestive of large crystal plates, although free cementite is
absent. This effect is due to the tendency of tliis eutectic
already noted (p. 185) to form "colonies" of definite parallel
orientation.

Grey cast-iron contains, as its principal constituents,
pearlite, graphite, and either ferrite or cementite, according
as the proportion of combined carbon is below or above the
' C. A. Edwards, J. Iron Steel fust., 1910, ii. 147.

378 METALLOGRAPHY

eutectoid proportion. A further constituent, which is rarely
absent from the commercial varieties of grey iron, is the
eutectic of iron and iron phosphide, Fe^P, which forms
reticulated masses, brilliantly white in an etched specimen,
and appearing as distinct islets of hard material in the softer
ground-mass.

A Modified Equilibrium Diagram

It must be admitted that the assumption of a metastable
and a stable equilibrium of iron and carbon, coinciding almost
exactly in the higher range of temperature, is far from satis-
factory. It has, nevertheless, been adopted by the majority of
metallographists as the best available expression of the facts.
A provisional hypothesis has recently been proposed,^ which
avoids many of the difficulties of the double diagram. Upton
provides for the inclusion of cementite in the stable system,
whilst confining its stability to a range of temperature limited
in both directions. The following facts, neglected by other
investigators, have been utilized : —

1. The thermal observations of Carpenter and Keeling'
show arrests in almost all the alloys examined, at 800° and a
little above 600° respectively. Attempts have been made to
explain away these arrests, but there is every reason to suppose
them to be real.

2. Cementite decomposes above 800°, setting free graphite.''
Even after prolonged heating at 900°, a part of the carbon
remains combined or dissolved, amounting to about 3*6 per
cent.*

3. Long annealing of cementite below 600° leads to the
formation of a higher carbide, which may be separated

' G. B. Upton, y. Physical Chein., 1908, 12, 506 ; 1909, 13, 388.

' J. Irott Steel Inst., 1904, i. 224.

' F. Mylius, F. Foerster, and G. Schoene, Zeitsch. anorg. C/iem,, 13,
38 ; Ber., 1896, 29, 2991 ; J. O. Arnold and A. A. Read, Trans. Chem.
Soc, 1894, 6S, 788.

■■ E. H. Saniter, Metallographist, 1902, 5, 215.

METALLOGRAPHY OF IRON AND STEEL 379

mechanically and chemically. The formula of this carbide
appears to be FejC

4. The iron in equilibrium with graphite in grey cast-irons
can hardly be pure ferrite, its magnetic susceptibility, allowance
being made for the graphite present, being far below that of
pure iron.

5. The combined carbon in slowly cooled cast-iron is
increased by heating between 800° and 1000°.'' This is
contrary to the view that cementite is metastable at all
temperatures.

6. The brittleness produced in steels by heating at 500-
550°' indicates a change of constitution at a low temperature.

Utilizing these and other facts of observation, Upton has
constructed the diagram shown in Fig. 107. The liquidus and
solidus, and the transformations of steels, containing less than
©■89 per cent. C, remain as before. The only solid phases
separating from the liquid, under conditions of equilibrium,
are graphite and a solid solution of carbon in y-iron, here
denoted by I. Below the solidus, there is a small range of
temperature within which these are the stable phases. At
about 1095°, they react to form a carbide containing 3'5 per
cent. C, and therefore represented by the formula FegC.
Upton suggests that this may be only a solid solution, and not
a definite compound j but this would involve a considerable
modification of the diagram, as in that case the transformation
line at 800° could not be horizontal, but must have a consider-
able inclination. At 800°, the carbide constituent gives up a
part of its iron, and passes into cementite, and at 615° a further
quantity of iron, now transformed into the a modification, is
given up, and the stable carbide YtjZ is formed. The last
reaction proceeds very slowly, and equilibrium is not attained
until after prolonged annealing.

' E. D. Campbell and M. B. Kennedy, J. Iron Steel Inst , 1902, ii,
2S8 ; C. Benedicks, of. cit.

- E. Heyn and O. Bauer, Milt. k. Mat.-Priif. Ami., 1906, 24, 29 ;
Stahlu. Eisen, 1907, 27, 1565 ; G. P. Royston, J. Iron Steel Inst., 1897,
i. 166.

= J. E. Stead, J. Iron Steel Inst., 1898, i. 145 ; ii. 137 ; C. E.
Stromeyer, ibid., 1907, i. 200.

38o

METALLOGRAPHY

Undercooling readily takes place in the solidification of the
molten cast-irons, so that the left-hand branches of the liquidus
and solidus are prolonged into the FegC region. The structure

1500

^\

14 00

\

ISOO

s

\ \ ^ '' Graphite

ISOO

I

\ \ ^^ + lijuid

neo

/ I + \ Graplitte

1

fooa

/ I + r.,c

Te^ C + Grcuphite

900

BOO

\

^

t + Fe^ C.

Fe_,C +

Ora.jthite

<x + Fej C

600

500

a Ferrite * Fe^ C

F»^C

+

400

Graphite

I a 3 4 s e 7 a a lo ii }ic
Fig. 107. — Upton's diagram of the iron-carbon alloys.

of white cast-irons, on this assumption, is due to the presence
of crystals of I (white) and masses of undercooled mother-
liquor (dark on etching). This is the weakest point in Upton's
explanation, for it is difficult to believe that the beautifully
defined structure found in white cast-irons (Plate XIV., A and
B) is not a true eutectic.

When the white iron is annealed, the first reaction is assumed
to be that which was suppressed during solidification, namely,

METALLOGRAPHY OF IRON AND STEEL 381

the formation of graphite. Hence the production of temper
carbon. Further heating at temperatures below 1095° should
cause the re-solution ©f a part of this carbon. The statement of
Goerens and Gutowsky, that all graphite produced in or near the
freezing range is formed by the decomposition of cementite, is
denied, and it is afifirmed that the parent substance is in all cases
a solid solution (austenite or I) and not cementite. A thorough
investigation of the microscopic structure, using reagents cap-
able of distinguishing between cementite and supersaturated
austenite or its decomposition-products, is desirable. Means
of distinguishing the three supposed carbides have also to be
devised.

A complete discussion of the proposed diagram is impos-
sible in the space available. Whilst the scheme is clearly
defective in some respects, it represents a bold and original
attempt to solve a difficult problem, and contains some features
which will almost certainly be incorporated in the complete
diagram of future metallographists. Further suggestions for a
modified diagram are given by A. Kroll ' in a memoir which
contains important new facts.

The Influence of other Elements on the
Iron-Carbon System

Commercial varieties of iron and steel commonly contain
other elements besides iron and carbon, frequently in notable
proportions. The addition of a third element may affect the
equilibrium of iron and carbon in several ways. In the first
place, the tendency of the alloys to assume the metastable
(carbide) form rather than the stable (graphitic) form depends
to a great extent on the presence of other elements. Silicon
greatly accelerates the separation of graphite, whilst manganese
and sulphur favour the retention of carbon in the combined

• J. Iron Steel Inst., 1910, i. 304.

382 METALLOGRAPHY

form.' The quantitative relationships have been most fully
worked out in the austenite-cementite series, and the conclusion
has been reached that two principal types of, ternary systems
present themselves.^

1. The third element forms solid solutions with y-iron, and
also enters into the solid solutions containing cementite.
Equilibria of this kind are obtained with Mn, Si, Cr, Ni,
and W.

2. The third element forms a compound with iron, which
has only a small solubility in solid y-iron, and is insoluble in
cementite. A ternary eutectic is therefore formed. Examples
of this type are furnished by P, As, Sb, and Sn.

Manganese may be taken as a typical representative of the
first class. Manganese is isomorphous with y-iron, and austenite
is in fact best obtained by quenching iron containing manganese
(p. 373). Manganese carbide, MnaC, is isomorphous with FeaC,
which it closely resembles. The structure of a white cast-iron
is thus unchanged by addition of manganese, and the freezing-
point curve is only slightly modified, the general form remaining
the same. The exceedingly tough manganese steel manu-
factured contains 12-13 per cent. Mn and about i per cent. C.^
Alloys containing so large a quantity of manganese do not
undergo transformation to pearlite on cooling, that is, austenite
rich in manganese is stable even at the ordinary temperature.^

In the iron-carbon-silicon alloys, two series of solid solutions
are formed, one being an austenite containing carbon and silicon
dissolved in y-iron, and the other a solution of FejC and FcaSi.'*

The behaviour of alloys containing phosphorus is very
different.'' The ternary system FeFesC-FesP resembles such

' The factors for calculating the displacement of graphite by other
elements have been worked out by M. Orthey, Metallurgie, 1907, 4, 196.

" P. Goerens, Metallurgie, 1909, 6, 531, 537.

' R. A. Hadfield, Proc. Inst. Civ. Eng., i888, 93, iii. i,

' L. Guillet, Compt. rend., 1908, 146, 74 ; F. Wiist, Metallurgie, 1909,
6, 3- The structure of the ternary alloys of iron and carbon has been investi-
gated by L. Guillet, and summarized in Rev. de Metallurgie, 1905, 2, 350.

' W. Gontermann, Zeitsch. anorg. Chem,, 1908, 59, 373.

° J. E. Stead, J. Iron Steel Inst., 1900, ii. 109 ; P. Goerens and
Dobbelstein, Metallurgie, 1908, 6, 561 ; F. Wiist, ibid., 73.

METALLOGRAPHY OF IRON AND STEEL 383

a simple system as that of lead, tin, and bismuth. y-Iron dis-
solves about 2 per cent. C and 2 per cent. P, but iron carbide
and phosphide are mutually insoluble. There are thus three
binary eutectics (austenite-cementite, austenite-phosphide and
cementite-phosphide) and a ternary eutectic point, at which all
three solid phases are in equilibrium. The latter point lies at
953° and gi's per cent. Fe, 2'o per cent. C, and 67 per cent, P.
Arsenic, antimony, and tin form similar systems.^

The effect of these added elements on the transformations
of carbon steels below the solidus is naturally exceedingly
complex, and the metallography of the ternary and quaternary
steels, many of which have great technical importance, has
already become the subject of several special treatises.^

Meteoric Irons

We may conclude this chapter with a brief account of the
metallography of the specimens of iron and its alloys of
extra-terrestrial origin, known as meteoric irons, or as
meteorites. The historical interest of this branch of the
subject is considerable, for it was in the study of meteorites
that the plan of examining polished and etched surfaces was
first introduced by Widmanstatten, and it was from the same
study that Sorby was afterwards led to investigate the
behaviour of terrestrial iron and steel under similar treat-
ment."

The principal meteoric irons are composed of iron and
nickel. They may be either octahedral or cubic in crystalline
structure. Considering the octahedral irons first, a coarse
structure is readily developed by etching or heat-tinting.
Three principal constituents are observed : —

Kamacite is a solid solution of a-nickel in a-iron, contain-
ing 6-7 per cent. Ni, and forms thick plates, built up of cubes
twinned on the fluorite plan. Cubic meteoric irons consist
wholly of unsaturated kamacite. Neumann's lines appear in
kamacite after mechanical deformation.

' P. Goerens and K. EUingen, Metallurpe, 1910, 7, 72.

« See, for instance, L. Guillet, Les Aciers Spkiaux., Paris, 1908.

' See p. 3.

384 METALLOGRAPHY

Taenite is a solution of iron in a-nickel, containing i3~3S
per cent. Ni.

Pkssite has a duplex structure, and is obviously a eutectic
or eutectoid mixture of kamacite and taenite. The particles
may be intermixed, or they may reproduce, on a small scale,
the general octahedral structure of the meteorite.

The problem presented by these alloys is that of reconciling
their structure with that of terrestrial alloys of iron and nickel.
These metals form solid solutions, y-iron being isomorphous
with yS-nickel, and a-iron with a-nickel. The transformation is
not reversible for all the alloys, a considerable lag of tempera-
ture (temperature hysteresis) being observed in alloys contain-
ing from o to 25 per cent. Ni; but the microscopic structure is
in all cases homogeneous, the alloys being composed wholly of
polyhedra. Osmond and Cartaud considered that a eutectoid
point existed at 25 per cent. Ni (Fig. 108), and that the
absence of any corresponding structure in the artificial alloys
was due to the comparatively rapid cooling of the specimens.^
It was observed, however,^ that certain meteorites had a
granular, instead of the usual orientated, structure, and that
normal meteorites could be rendered granular by heating to
950°. Following up this clue, it was found that the structure
of meteorites could be destroyed by annealing.' Octahedral
irons, packed in magnesia enclosed in a layer of reduced iron,
heated to 800°, showed granulation in the kamacite area after
I minute, and after 15 hours at 1000° the plessite and taenite
had almost completely disappeared, and the kamacite had
become granular throughout. The complex structure was in
this way converted almost completely into the polyhedra
characteristic of the terrestrial alloys.

The octahedral structure of the irons, whatever be its
nature, must have arisen from crystallization in a solid
solution, for the uniform orientation throughout a large
specimen is inconsistent with crystallization from the liquid

' Rev. de MHallurgit, 1904, 1, 69.

"^ F. Berwerth, Sitzungsber. h. Akad. Wiss. IVten, 1905, 114, i. 345 ;
F. Rinne and H. E. Boecke, N. Jahrb. Min., 1907 [Festband], 254.
» W. Fraenkel and G. Tammann, Zeitsch. anorg. Chem., 1908, 60, 416,

METALLOGRAPHY OF IRON AND STEEL 385

state. These experiments, and others of a similar character,
led to the conclusion that the structure of meteoric irons was
an unstable one, and that the final state of equilibrium was
one of homogeneous solid solution. It has now, however,
been shown that these results are to be explained by the fact
that the temperature of annealing lay far above the eutectoid

700

K

1 F»

t

fi Ni

600

\

\

^ -...^

SOO

\

\^

/

^^\

400

\

Irrl

ver^iXlB "-./

RevtrtxbXt >v

300

\

\

200

n Fe

\

a Nt

+

/OO

+

\

I /

" F«

\/

tQQ

200

\

fO ZO do 40 CO to 70 SO 00 fOO ?a Ni

Fig. io8. — Iron-nickel alloys.

point. When an alloy of iron with 12 per cent, of nickel is
prepared by the aluminothermic process, and is cooled very
slowly from 350° to the ordinary temperature, plessite is
formed, and the structure of the octahedral irons is repro-
duced.' The difficulty experienced in obtaining a structure
similar to that of the natural iron . is accounted for by the
very low velocity of diffusion in the solid solution below

35°°-

' C. Benedicks, Rev. de MStallurgie, 191 1, 8, 85.

T.P.C.

2 G

CHAPTER XVIII

THE METALLOGRAPHY OF INDUSTRIAL ALLOYS

The possible types of structure encountered in the laboratory
study of alloys are extremely numerous, but only a limited
number of these possess properties which render the alloys
suitable for technical application. Many structures of metallo-
graphic interest are associated with great mechanical weakness,
rendering the alloys unfit for any purpose involving resistance
to mechanical stress, whether gradually or suddenly applied.
Inter-metallic compounds, and the solid solutions which they
form with their components, are in general too hard and
brittle to find technical application except in a few special
cases, as, for instance, when a brilliant, mirror-like surface is
required, as in speculum metal. Resistance to shock is not
demanded in such a case, and the extreme brittleness of the
alloy is therefore no disadvantage. Occasionally a certain
appearance or colour is required for decorative purposes, as
in jewellery, and other properties may be comparatively
unimportant.

A large proportion of the alloys employea for constructional
purposes, and therefore required to possess certain physical
and mechanical properties, fall into a few well-defined groups.
For instance, amongst the alloys of copper with other metals,
the a solid solutions, which consist of copper holding a second
metal in a state of solid solution in quantities insufficient to
saturate it, are some of the most important of industrial alloys.
Whether the second metal be zinc, tin, aluminium, nickel, iron,
or manganese, the resultant alloys, within certain limits of
composition, bear a close resemblance to one another. The

386

METALLOGRAPHY OF INDUSTRIAL ALLOYS 387

microscopic structure is in all cases the same, th^t of a
crystalline solid, built up of polygonal grains which are homo-
geneous if slowly cooled or annealed, but exhibit a " cored "
structure if cooled too rapidly for the attainment of equilibrium.
Mechanically, all these alloys are characterized by toughness
and malleability, in degrees which naturally vary with the
nature and quantity of the added metal.

A second type of structure, due to the separation of a
micrographic constituent from solid solution during cooling,
also presents itself in several instances among the copper
alloys. This " a/3 " structure is very characteristic, and the
resemblance between alloys of this type containing different
metals is often very close, so that an alloy of copper containing
8 per cent, of aluminium, for instance, is almost indistinguish-
able in appearance, so far as the arrangement of the a and |3
phases in the micro-section is concerned, from one containing
42 per cent, of zinc. Here, again, the similarity of structure
corresponds with a marked similarity of properties.'

In this chapter, some of the more important alloys which
find industrial application are briefly reviewed. An entirely
systematic arrangement has not been adopted, so that whilst
the grouping is generally according to chemical composition,
alloys of different composition have occasionally been grouped
together, if the similarity of their application seemed to render
such an arrangement convenient. The list of alloys mentioned
below could be extended indefinitely did space permit, but the
principal types of structure at least receive mention.^

Copper-zinc. — The industrial alloys of copper and zinc fall,
with a few unimportant exceptions, into two classes, those
composed of the a solution only, , and those built up of the
a and /8 phases in conjunction. The first group includes the
brasses, and the second Muntz's metal and similar alloys.

Of the brasses, an alloy containing 33 per cent. Zn is

' The resemblance is even closer than at first appears. The ^-solution
in the case of the alloys of copper with tin and with aluminium is resolved
into two phases at low temperatures, and it has now been shown that a
similar condition occurs in the alloys of copper and zinc (see p. 62).

' The composition of a large number of industrial alloys is tabulated by
\\. Kaiser, Metailurgie, 191 1, 8, 257, 296.

388 METALLOGRAPHY

commonly used for casting purposes, whilst the maximum
ductility is possessed by the alloy containing only 30 per cent.
Zn. The latter mixture is capable of withstanding extremely
severe mechanical deformation, and is therefore employed m
the production of tubes and wires by drawing through dies
at the ordinary temperature. The destruction of crystalline
structure in the drawing process is very extensive, and the
formation of the amorphous modification gives rise to such
an increase of hardness and brittleness that it is necessary to
carry out the reduction in thickness in a number of successive
stages, restoring the crystalline structure by annealing after
each reduction. The formation of a cartridge-case «from a
disk by pressing between dies is a typical example of the
severe mechanical treatment applied to a 70 : 30 brass. The
microscopic structure of such a worked metal is highly con-
fused, the u. crystals being broken down and thrust into and
through one another by the pressure. The annealing is best
performed at 600-650°, when recrystallization takes place.
Twinned u. crystals are abundantly produced, and the size
of the crystals increases both with the temperature and with
the duration of the annealing process.^ The " burning " of
brass above 750° is probably due to volatilization of zinc
from the a phase. The a crystals are homogeneous through-
out, " cores " only being observed in the cast metal, and
disappearing completely after working and annealing. An
example has been shown in Plate II., B. Lead is often added
to cast brass to render it easier to cut or turn*; it is recognizable
under the microscope as isolated globules, mechanically retained
by the solidifying metal, and not truly alloyed.

The second group of copper-zinc alloys comprises those
containing from 36'5 to 46 per cent, of zirlc, and made up,
when in a state of equilibrium, of the a and j3 solid solutions.
From the diagram (Fig. 29), it will be seen that the /3 region
is of peculiar form, narrowing considerably as the temperature
falls.'' A number of alloys, therefore, composed after solidification

' See, for the mechanism of annealing in these and similar alloys, G. D.
Bengough and O. F. Hudson, y. Inst. Metals, 1909, 1, 89.

^ E. S. Shepherd, J. Physical C/iem., 1904, 8, 421 ; V. E. Tafel,

METALLOGRAPHY OF INDUSTRIAL ALLOYS 389

entirely of j8 crystals, become heterogeneous at lower tempera-
tures, either the a or the y phase crystallizing out, according
to the composition. Alloys containing the y phase in excess,
even in small quantity, are devoid of technical importance,
the y crystals, which appear to consist of the compound
CuoZnj, being extremely hard and brittle.

Muntz's metal, containing 40 per cent. Zn, is composed of
a and /3 crystals, but may be obtained in the form of homo-
geneous /3 crystals by quenching from temperatures above
750°. The structure of a rolled bar of Muntz's metal is seen
in transverse section in Plate IX., A, in which the light areas
represent the u, constituent and the dark the more readily
etched /3 constituent. If slowly cooled through the trans-
formation range, the a crystals assume characteristic forms,
and the structure is then almost indistinguishable from that
of the aluminium bronze shown in Plate VIII., B.

The /3 constituent gives to alloys in which it occurs the
property of being ductile at a higher temperature, so that
Muntz's metal, unlike brass, may be rolled hot. On the other
hand, the p crystals, or the mixture of ullramicroscopic particles
of o and y, are less ductile at the ordinary temperature than
the a. Muntz's metal quenched from above 750° is therefore
harder and stronger, but less ductile, than the annealed alloy.

The effect of small additions of a third metal on the
structure and properties of the ajS alloys has been already
referred to in connection with the work of Guillet (page 191).
An alloy containing 37 per cent, of zinc and i per cent, of tin
is used under the name of naval brass, on account of its
superior resistance to the corrosive action of sea-water. This
quantity of tin is entirely held in solid solution, and is not
microscopically recognizable. An alloy of similar structure,
containing about 40 per cent, of zinc, and i per cent, or rather
more of iron, is used in engineering under the name of Delta
metal. In this case also, the iron is held in solid solution.
Small additions of aluminium, silicon, and manganese are also
Metalhtrgie, 1908, 5, 343. See also G. Charpy, Etude des AUiages,
1901, I. A eutectoid is formed, but the finely dispersed, ultramicroscopic
mixture of o and 7 is relatively very stable, and does not exhibit segregation
unless an excess of either o or 7 is present.

390 . METALLOGRAPHY

made with the object of improving the mechanical properties
of the alloys of this group, while leaving the a/3 structure
intact. They are known collectively as special brasses, and
separately as aluminium brass, etc., although such erroneous
designations as manganese bronze, etc., properly belonging to
quite different alloys, are in frequent use/

Copper-tin. — Here, again, the first important series of alloys
consists of a solid solutions, comprising those mixtures contain-
ing up to about 12 per cent, of tin. The structure of such alloys
is unaffected by quenching. The best gun-metals contain from
8 to 10 per cent. Sn, and therefore fall within the limits of
this group ; they are characterized by great strength, elasticity,
and toughness.

In the microscopical examination of bronzes, crystals of
stannic oxide are often observed, mechanically enclosed in
the alloy, and representing a serious source of weakness.
Such enclosures are evidence of defective melting and
casting. Re-melting is frequently resorted to as a means
of eliminating this impurity.

Bronzes containing more than 12 per cent. Sn are also
employed. As will be seen from the equilibrium diagram
(Fig. 98), their structure depends on the heat-treatment they
have received. Above 500° they are composed of the a and j3
phases, a structure possessing great strength, and permitting
forging at temperatures within the a/3 range. This structure
can only be preserved by cooling rapidly from above 550°.
Slowly cooled alloys of the same composition do not contain
the /3 constituent, which breaks up, as described on p. 6 1, into
the eutectoid complex of the a and S phases, forming islands,
bounded by a well-defined band of the hard, bluish-white
8 constituent. This substance, sometimes regarded as a
definite compound, Cu4Sn, is a source of brittleness, and
alloys containing it are weaker than those of the same
composition, in which the a/3 structure has been preserved by

' The group which has been studied most thoroughly is that of the
aluminium brasses. See H. C. H. Carpenter and C. A. Edwards, Intern.
Zeitsch. Metallop-afhie, 1912, 2, 209 ; M. Levi-Malvano and M. JMaran-
tonio, Gazzella, 1911, 41, ii. 282.

METALLOGRAPHY OF INDUSTRIAL ALLOYS 391

quenching. The aS alloys containing up to 25 per cent. Sn are
known as bell-metal, and possess great hardness and sonority.

Bronzes containing both tin and zinc find extensive
application. In the construction of machinery, a8 bronzes
containing 10-18 per cent, of tin, and 2-4 per cent, of zinc,
are used. Lead is often added to pi'oduce cleaner castings
and to give greater facility in machining, but it is a serious
source of weakness, remaining undissolved as inter-crystalline
material, and thus greatly reducing the strength. "■ For
statuary purposes, the percentage of zinc is as high as 5,
that of tin being 5-8. The presence of zinc facilitates casting
and modifies the colour. Lead is also added, the effect of
this being not only to produce a highly fluid alloy, but to
modify very greatly the oxidation film, or "patina," which
forms on the surface. Some Japanese and Chinese art bronzes,
which assume a fine matt black patina, contain as much as
10-16 per cent, of lead, the brittleness thus produced being
unimportant for the purpose of statuettes. The production of
a patina, either by spontaneous oxidation in the air, or by
treatment with oxidizing solutions, is a phenomenon of great
interest, which has received very little attention from the
scientific point of view. Very small quantities of foreign
metals have a marked effect in modifying the colour of
this surface layer, as in the case of the Japanese alloy shakudo,
the fine patina of which is dependent on the presence of gold
in quantities varying from i to 4 per cent. The purple or
black surface coating is produced by "pickling" with a
solution, consisting generally of copper sulphate and acetate.^
The Japanese excel in the preparation of this kind of oxidized
surface layer.

Bronze coins, which are required to take a sharp impression
of the die and to possess sufficient hardness to resist the wear
and tear of handling, are a solutions, corisisting in this and
most other countries of 95 per cent. Cu, 4 per cent. Sn, and
r per cent. Zn.

Of alloys richer in tin, the only mixture of importance is
speculum metal, formerly used in the construction of reflecting
■ J. T. Milton, J. Inst. Metals, 1909, 1, 57, and discussion.
= W. C. Robeits-Austen, y. Soc. Arts, 1890, 38, 690.

393 METALLOGRAPHY

telescopes, but now commonly replaced for that purpose by
glass, coated electrolytically with silver or palladium. Speculum
metal consists largely of the white, brittle 8 constituent, associ-
ated with the a. solution in small quantity, or, if containing
more than 32 per cent. Sn, with the compound CUjSn.

Copper-aluminium. — The aluminium bronzes are assuming
a continually increasing industrial importance. As before,
the a solid solution has good mechanical properties. It is
the only constituent of alloys containing less than 7-3 per
cent, Al, and the structure of such alloys is unaffected by
quenching. When the proportion of aluminium is increased,
alloys are obtained, the structure of which is dependent in the
highest degree on the heat-treatment, the concentration of the
solid solutions a and ^ varying with the temperature above
550°, and the |3 phase undergoing a change into a and y at
that point, as in the copper-tin alloys already referred to.'
The strongest alloy of the series contains 10 per cent, of
aluminium, and has a colour closely resembling that of gold.
The mechanical properties of this alloy, and of those contain-
ing a slightly smaller quantity of aluminium, approach more
nearly to those of steel than any other non-ferrous alloy.
Mixtures containing the y-constituent are brittle, but as this
phase is produced only by long heating at 400°, alloys contain-
ing up to II per cent. Al may be employed at the ordinary
temperature. The a/3 structure of the 10 per cent, alloy is
shown in Plate VIII., B.

A few alloys at the other end of the range, containing
only small quantities of aluminium, will be described under
aluminium. Mixtures containing from 12-92 per cent, of
aluminium are without industrial importance.

' The copper-aluminium equilibrium diagram is very complicated, and
its form in the region 8-30 per cent. Al is still a matter of controversy.
The most important papers are : H. C. H. Carpenter and C. A. Edwards,
Proc. Jnst. Mech. Eng., 1907, 57, and discussion thereon ; B. E. Curry,
J. Physical Ckem., 1907, 11, 425 ; B. E. Curry and S. H. Woods, ibid.,
461 ; A. G. C. Gwyer, Zeitsch. anorg. Cliem., 1908, 67, II3 ; L. Guillet,
Reo. de Mitallurgie, 1905, 2, 567. The most careful determinations are
those of Carpenter and Edwards, and of Curry.

METALLOGRAPHY OF INDUSTRIAL ALLOYS 393

Phosphor-bronze.— T\i!t addition of phosphorus to copper is
practised for the purpose of removing oxygen from the molten
metal. The quantity used must not exceed that required for
complete deoxidation by more than about o'l per cent., a
larger quantity causing brittleness, owing to the separation
of hard particles of the phosphide, CU3P. Such a mixture is
sometimes, very incorrectly, termed phosphor-bronze. The
true alloys of this name consist of copper, tin, and phosphorus,
the best composition being from 91-92 per cent. Cu, 7 ■4-8' 7
per cent. Sn, and o*3-o'6 per cent. P, except when used as
a bearing metal, when a different structure is required.
Malleable phosphor-bronze of the above composition is an
u, solution, and does not differ in structure from an ordinary
gun-metal. The presence of phosphorus in solid solution
hardens the metal, but does not otherwise affect the properties
to any very marked extent. The improvement in quality of
gun-metal containing a little phosphorus is to be attributed
mainly to the complete removal of oxides.^

Another class of phosphor-bronze alloys, employed in a
cast state and not required to possess malleability, contains
larger quantities of both tin and phosphorus, the. former
element being present to the extent of 10-15 P^"^ cent., and
the latter to the extent of 07-1 '5 per cent. Such alloys are
very hard, and are able to resist friction ; they are therefore
employed in the manufacture of gear-wheels, bearings, etc.
The hardness is due to the presence of the 8 constituent of
the copper-tin series, and of the hard copper phosphide CujP.
These two substances, forming isolated masses in the com-
paratively soft matrix of a solution, produce the combination
of great hardness with a limited, but distinct, plasticity which
is required for such purposes.

Other alloys of copper. — Several metals, capable of entering
into solid solution in copper, yield alloys of industrial impor-
tance. Nickel, as has been shown on p. 48, is isomorphous
with copper, and does not form a compound with it. The
whole series, from pure copper to pure nickel, may therefore
be regarded as consisting of the a solution. In accordance
' A. Philip, J. Inst. Metals, 1909, 1, 164.

394 METALLOGRAPHY

with the general rule for solid solutions, the hardness increases
very greatly towards the middle of the series, so that the alloys
of greatest use are those containing comparatively small
quantities of nickel. The alloy containing 2-3 per cent. Ni
has been found valuable in locomotive work, having great
toughness at the temperature of the fire-box. This property
of resisting high temperatures without serious deterioration,
is common to most of the o solutions of copper in which the
quantity of added metal is not large. Of alloys richer in
nickel, that containing 40 per cent. Ni has a low electrical
conductivity, the temperature-coefficient of which is practically
zero. As it is capable of being drawn into wire, it is largely
employed in the construction of resistance coils, under the
name of constantan.

Manganese also forms a continuous series of solid solutions
with copper, the middle members of which are hard and some-
what brittle. The alloys containing 2-4 per cent. Mn are
harder and tougher than copper, and like the alloys with
nickel, retain their properties at comparatively high tempera-:
tures. The term " manganese-bronze," which should be
retained for these alloys, is often improperly applied to
copper-zinc alloys of the Muntz's metal type, to which small
quantities of manganese, usually accompanied by iron, have
been added with the object of removing oxygen and of
conferring increased strength on the alloys, which should
properly be termed " manganese brass." The true man-
ganese bronzes are a solutions. Iron also forms a. solu-
tions with copper, characterized by great toughness and
a hardness exceeding that of the pure metal. An alloy
containing 82 per cent. Cu and 15 per cent. Mn, the remainder
being nickel and iron, has a low electrical conductivity and
almost zero temperature-coefficient, and is hence used for
electrical resistance coils under the name of manganin. All
the components are in a state of solid solution.

Ternary alloys of copper, zinc, and nickel, forming a single
solid solution, are much employed under the names of German
silver and various other trade designations. The most useful
alloys of the series contain 50-60 per cent. Cu, 15-20 per cent.

METALLOGRAPHY OF INDUSTRIAL ALLOYS 395

Ni, and 20-30 per cent. Zn, but variations even beyond these
limits are found. Platinoid, an alloy used in the construction
of electrical resistances, is a German silver containing small
quantities of tungsten.

Alloys of silver. — Silver, when employed for the purposes
of coinage, or of the manufacture of silver articles, is always
alloyed with a metal capable of entering into solid solution
with it, in order to give increased hardness and resistance to
wear. Sterling silver, used in the British coinage and for
ornamental and other work bearing the hall-mark, consists of
92'5 per cent, silver, and 7*5 per cent, copper. The " fineness "
of silver and gold being reckoned in thousandths instead of in
percentages, sterling silver is said to be 925 fine. It is not a
homogeneous solid solution^ but consists of crystals of a solid
solution rich in silver, separated by a eutectic (see Fig. 23,
p. 57). This fact renders the alloys liable to separate during
solidification, so that different portions of the ingot contain
different quantities of the eutectic, a fact of great importance
to the assayer.i Alloys containing only 90 per cent. Ag and
even less, find application in jewellery. Cadmium is sometimes
added in small quantities. The standards vary considerably
in other countries.

The other alloys of silver are only of limited importance,
the solid solution containing platinum being the only one of
marked interest.

Alloys of gold. — Fine gold, being a very soft metal, is not
in general use, and a hardening metal is almost invariably
added. The added metal is either copper or silver, both of
which form homogeneous solid solutions with gold, British
standard gold contains 91 '67 per cent, gold, and 8-33 per cent,
copper ("22 carat" = ff =^ 91-67 per cent.), and this is
the composition employed in coinage, whilst most of the
alloys used for the manufacture of jewellery contain larger
quantities of the hardening metal. Silver may take the
place of copper, yielding alloys of paler colour. In both
cases the structure is homogeneous.

1 \V. C. Roberts (afterwards Roberts-Austen), Pioc. Roy. Soc, 1875,
23, 481.

396 METALLOGRAPHY

Alloys of aluminium.— TYiQ great lightness of aluminium
has led to the desire to introduce its use as a structural metal
in motor-cars and other machines in which the saving of weight
is aimed at, but its comparatively poor mechanical properties,
and the difficulty of obtaining sound castings, have proved to
be obstacles to its use. Solid solutions, of which aluminium
is the principal constituent, have given much better results in
practice, and are now extensively employed. These so-called
"light alloys" contain up to 20 per cent, of zinc, 6 per cent,
of copper, or 5 per cent, of nickel, or proportionate quantities
of two or more of these metals. Magnalium is an aluminium
alloy containing 1-2 per cent, of magnesium, generally together
with similar quantities of copper or nickel. Duralumin con-
tains about 3'S per cent, of copper, together with 0-5 per cent,
or less of magnesium. Its properties may be modified in a
remarkable manner by heat treatment.^

Alloys of lead and tin. — Ordinary plumbers' solder consists
of 67 per cent, of lead and 33 per cent, of tin ; it is therefore
composed of crystallites of lead, surrounded by the eutectic of
the two metals. The structure is developed by polishing very
lightly with chromic oxide. Pewter contains 8-15 per cent, of
lead, the remainder being tin. It consists partly of a solid solu-
tion of lead in tin and partly of eutectic. Hard lead, used for
acid-pumps, etc., consists of lead containing 10-15 per cent,
of antimony, whilst the alloys containing up to 20 per cent,
of antimony are also employed as type-metal. These alloys
are composed of crystallites of antimony, embedded in a large
quantity of the eutectic of the two metals. Britannia metal
consists of tin, with 5-8 per cent, of antimony and occasionally
a little lead j it consists essentially of a solid solution.

Bearing metals. — The bearing metals, or anti-friction alloys,
may be considered together, in spite of the diversity of compo-
sition which they present, on account of the general similarity
of their micrographic arrangement of constituents. The requi-
site properties are possessed by alloys consisting of a hard and
a soft constituent, so distributed that the soft material provides
the necessary plasticity, whilst the harder substance receives

' A. Wilm, Aletallurgie, 191 1, 8, 225.

METALLOGRAPHY OF INDUSTRIAL ALLOYS 397

the load and resists abrasion. Two methods of obtaining the
required combination of properties are met with. According
to the first, crystallites of one or more hard constituents are
distributed through a soft ground-mass or matrix, whilst the
second consists in preparing a sponge, the meshes of which are
tough and moderately hard, filled up with a soft, plastic material.
The first class includes the larger number of bearing metals.

The soft matrix is either lead or tin, or an alloy of these
metals, containing a eutectic. The hard constituent is com-
posed very frequently of the cubical crystals of tin and
antimony, corresponding approximately with the formula
SbSn (Plate VII., B). Binary alloys of lead and antimony
are sometimes used, the crystallites of antimony in this case
serving the same purpose. More frequently, all three metals
are employed, in such proportions that the cubic constituent
is present. The range of composition of such mixtures is
very wide, and representatives of almost every member of
the class are met with in practice. The addition of copper
in small quantities provides an additional hard constituent,
compounds of copper with tin or antimony separating in six-
rayed stars of extreme hardness.^ The rays have frequently a
remarkable hollow structure, each ray resembling an elongated
hexagonal box. The hard copper constituent becomes apparent
in relief after polishing on a soft bed, without etching, whilst
the cubes of tin and antimony do not make their appearance
until after the surface has been etched. Lead has also been
hardened by means of sodium, the hard constituent being the
compound NaaPbg.

In the second class of bearing metals, the "plastic
bronzes," the spongy mass is composed of copper hardened
by means of tin, antimony, or nickel, the plastic filling
material being chiefly lead. These alloys are obtained by
the solidification of copper-lead emulsions, as although tin
is added to increase the miscibility of the copper and lead,
its quantity (about 6 per cent., against 25-30 per cent, of lead)

' These crystallites are stated by Charpy, Btude des Alliages, 225, to
consist of CusSn and CUjSb respectively, but such compounds are obviously
incapable of existence in the presence of an excess of tin. The formulae of
the compounds in question remain uncertain ; they must be rich in tin.

398 METALLOGRAPHY

is insufficient to bring about the formation of a homogeneous
liquid mixture. Under the conditions of casting, remarkably
little separation of the two constituents of the emulsion takes
place, and there is little segregation in the castings. Micro-
scopically examined, the spongy structure of the plastic
bronzes is clearly seen, the lead being enclosed in globules,
without any trace of crystalline outline.

Brittle alloys. — A large number of alloys are produced
commercially for the purpose of facilitating the addition of
small quantities of elements to molten metals. The method of
preparation frequently adopted is the reduction of the mixed
oxides by means of aluminium. Silicon and phosphorus as
such are never added to metals, rich alloys with iron, copper,
or tin being usually employed. These rich alloys, composed
principally of inter-metallic compounds, or of silicides, phos-
phides, etc., are commonly highly brittle, a property which is
advantageous when it is desired to weigh definite small
quantities for regulating the addition. Their microscopical
examination is, however, beset with difficulties, owing to the
fragility of the surfaces prepared by grinding. The process
of preparing a flat section by means of emery leads to the
breaking out of small particles, producing a rough, pitted
surface, on which it is impossible to distinguish structure.
Better results may often be obtained by employing a smooth
carborundum block for grinding in place of emery paper.

The technical phosphor-copper, containing lo per cent. P,
consists of crystals of the phosphide, CugP, surrounded by
eutectic, and yields good micro-sections (see Plate IV.).
Another quality, however, contains practically 15 per cent. P,
and consists only of the phosphide, with traces of a second
compound richer in phosphorus ; it is • almost impossible
to polish. So, only the lower cupro-silicons lend themselves
to microscopical examination. The metallography of these
brittle alloys is still very imperfectly developed. The
mechanical properties of alloys intended only for use as
components of mixtures are, of course, unimportant, but a
means of making a rapid metallographic examination is
desirable, in order to determine the degree of purity of the
alloy without recourse to a tedious chemical analysis.

APPENDIX

In the following tables the binary and ternary systems, of which
equilibrium diagrams have been published, are collected. The
probable formulae of inter-metallic compounds, when present, have
been added, with some notes as to their position in the diagrams.
Compounds, the evidence for which is unsatisfactory, are marked
with a ? The systems in each class are arranged in alphabetical
order of the symbols, each system appearing only once.

The references are not intended to form a complete biblio-
graphy. The reference placed against each system is to the most
recent complete study of that system, or, when there is reason to
reject an investigation as untrustworthy, to an earlier but more
accurate investigation. In order to economize space, the following
abbreviations have been used : —

J. P. = y. Physical Chem.
M. = Metallurgie.

R.M. = Rev. de Mdtallurgie.

Z.A. = Zeitsch. anorg. Chem.

Z.P. = Zeitsch. physikal. Chem.
A.A.L. = Atti. R. Accad. Lincei.
I.Z.M. = Intertiat. Zeitsch. Metallographie.

EUTECTIFEROUS SERIES

(Inter-metallic compounds being absent.)
,The freezing-point curve of these systems has a V form. The components
may crystallize in a pure state, or the crystals may retain a limited
quantity of the other component in solid solution. The molten metals
are miscible in all proportions.

Eutectic.

System

Atomic
per cent.

Temp.

Solid solutions.

Reference.

Ag-Bi

4-6 Ag

262°

Ag dissolves 3 atom.
%Bi.

G. J. Petrenko,Z.^., 1906,
SO, 133.

Ag-Cu

60 Ag

778

8 and 97 atom. %
Cu.

K. Friedrich and A. Le-
roux, M., 1907, 4, 293 ;
W.vonLepkowski,Z.^.,
1908, 59, 285.

Ag-Na

Near
rooNa

Very little.

E. Quercigh, Z.A., rgio,
68, 301 ; C. H. Mathew-
son,I.Z.M., 1911, 1, 51.

400

METALLOGRAPHY

EUTECTI FERGUS SERIES — con/ittHed.

Eutectic.

Reference.

System.

Atomic
per cent.

Temp.

Solid solutions.

Ag-Pb

4Ag

305'^

—

K. Friedrich, M., igo6, 3,

396.
G. Arrivaut, Comfit, rend. ,

Ag-Si

17 Si

800

—

1908, 147, 859.

Ag-Tl

SAg

287

Ag dissolves 6 atom
% Tl.

G.J.Petrenko,Z.^.,iQo6,
50,133.

Al-Si

9-sSi

577

W. Fraenkel, Z.A., 1908,
58, 154.

Al-Sn

100 Sn

232

The extent to which
solid solutions are
formed is uncertain

A. G. C. Gwyer, Z.A.,
1906,49,311 ;E.S. Shep-
herd,/..?., 1904,8,233.

Au-Bi

18-9 Au

240

Au dissolves 4 atom.
%Bi.

R. Vogel, Z.A., 1906, 50,
145.

Au-Co

72-9 Au

997°

W.Wahl,^.^., 1910,66,60.

Au-Ni

48 Au

950

Limits uncertain.

M. Levin, Z.A., 1905, 45,
238.

Au-Tl

27 Au

132

—

M.Levin,Z.^., 1905,45,31.

Bi-Cd

55-5 Cd

146

A. Portevin, S.M., 1907,
4, 389 ; A. Stoffel, Z.A.,
1907, 53, 137.

Bi-Cu

100 Bi

268

—

K. Jeriomin, Z.A., 1907,
55, 412.

Bi-Hg

Near
100 Hg

—

—

N. A. Pushin, Z.A., 1903,
36, 201.

Bi-Pb

S.6-5 Bi

125

Lead dissolves Bi to
a considerable ex-
tent.

A. Kapp, Ann. Fhysik.,
1901, [iv.] 6, 7S4.

Bi-Sn

43 Bi

137

—

A. Kapp, Ann. Physik.,
1901, [iv.] 6, 754.

Cd-Pb

67 Cd

249

Lead dissolves about
4 atom. % Cd.

A. Kapp, Ann. Physik.,
1901, [iv.] 6, 754.

Cd-Sn

67-6 Sn

177

Sn dissolves loatom.
%Cd.

A. P. Schleicher, I.Z.M.,
1912, 2, 76 ; A. Stoflfel,
z:a., 1907, 63, 137.

Cd-Tl

70 Tl

203-5

—

N.S.KurnakoffandN.A.
Pushin,Z.^., 1902,30,86.

Cd-Zn

73 Cd

270

—

G. Hindrichs, Z.A., 1907,
55, 415.

Hg.Pb

Near

—

Lead dissolves about

E. Janecke, Z.P., 1907,

100 Pig

30 atom. % Hg.

60, 399-

Hg-Sn

Near
100 Hg

—

—

W. J. v. Heteren, Z.A.,
1904, 42, 129.

Hg-Zn

I Zn

-42-5

Formed to an un-
known extent.

N. A. Pushin, Z.A., 1903,
36, 20 r.

Pb-Sb

21 Sb

228

A compound is
possibly formed in

A. V. Saposchnikoff and L
Kanewsky,/.j?«fj./'.4jj-.

*

the solid state.

Cheni.Soc.,i^&],%^,<jo\ ;
W. Gontermann, Z.A.,
1907, 65, 419.

APPENDIX
EUTECTIFEROUS SERIES — continued.

401

Eutectic.

Solid solutions.

-System.

Atomic
per cent.

Temp.

Reference.

Pb-Sn

75 Sn

180°

Lead dissolves tin.

W. Rosenhain and A. P.
Tucker, Phil. Trans.,

Sb-Si

Very limited range.

1908, 209a, 89 ; P. N.
Degens, Z.A., 1909, 63,
207; D.Mazzotto./.Z.AT.,
1911, 1, 289.
R. S. Williams, Z.A.,

SUSn

100 Sn

232

__

1907, 65, I.
S.Tamaru.Z.^., 1909,81,

Sn-Tl

31 Tl

170-2

_

40.
N. S.KurnakoffandN.A.

Sn-Zn

16 Zn

197

—

Pushin, .2.^. ,1902, 30,86.
F. Rudberg, Pogg. Ann.,
1830, 18, 240.

II

IsoMORPHOUs Series

A. The freezing-point curve lies entirely between the freezing-points

of the two components.

System,

Remarks.

Referemce.

Ag-Au

Ag-Pd
Au-Pd
Au-Pt

Bi-Sb
Co-Fe

Co-Ni

Cu-Ni

Cu-Pd
Cu-Pt
Fe-Mn

In-Pb
T.P.C,

The platinum end of
the curve is extra-
polated, but is pro-
bably correct.

The curve possibly
broken.

The a modifications
(magnetic) are also
isomorphous.

The a modifications
(magnetic) are also
completely isomor-
phous.

W. C. Roberts-Austen and T. Kirke

Rose, Proc. Roy. Soc, 1903, 71, 161.
R. Ruer, Z.A., 1906, 51, 115.
R. Ruer, Z.A., 1906, 51," 391.
F. Doerinckel, Z.A., 1907, 54, 333.

N. Parravano and E. Viviani, A.A.L.,

1910, [v] 19, i. 835.
W. Guertler and G. Tammann, Z.A.,

1905, 45, 205.

W. Guertler and G. Tammann, Z.A.,
1904, 42, 353.

W. Guertler and G. Tammann, Z.A.,

1907, 52, 25.
R. Ruer, Z.A , 1906, 61, 223.
F. Doerinckel, Z.A., 1907, 64, 333.
M. Levin and G. Tammann, Z.A.,

1905, 47, 136.
N. S. Kurnakoff and N. A. Pushin,

y. Puss. Phys. C/iem. 6'cc., 1906,88, 1 146.

2

402

METALLOGRAPH Y

B. The freezing-point curve passes through a minimum.

System.

Remarks.

Reference.

Au-Cu

Formerly supposed to

N. S. Kurnalcoff and S. F. Schemt-

be a eutectiferous

schuschny, Z.A., 1907, 64, 149.

series. Min. at 883°.

Co-Cr

Min. at 1340°.

K. Lewkonja, Z.A., 1908, 59, 293.

Cr-Ni

G. Voss, Z.A., 1908, 57, 34.

Cu-Mn

Min. at 865".

S. F. Schemtschuschny, G. Urazo6F, and
A. Rykowkoff, Z.A., 1908, 57, 253.

Fe-V

Min. near 1400°.

R. Vogeland G. Tammann, Z.A., 1908,
S8, 73-

Mn-Ni

Min. at 1000°.

S. F. Schemtschuschny, G. Urazofif, and
A. Rykowkoff, Z.A., 1908, 57, 253.

C. The metals form two series of solid solutions, separated by a gap.
The freezing-point curve presents a discontinuity.

Break.

System.

Atomic
per cent.

Limits of solid solutions.

Reference.

Au-Fe

45 Au

Very uncertain.

E. Isaac and G. Tammann,
Z.A., 1907, 53, 281.

Cd-Hg

65 Cd,

75 and 77 Cd at 190^.

H. C. BijI, Z.P., 1902, 41,

190°

65 and 80 Cd at 25°.

641.

Co-Cu

4 Co,

5 and 90 Co at 1107°.

K. Sahmen, Z.A., 1908,

1107°

57,1.

Cu-Fe

2 Fe,

3 and 97 Fe at iioo".

R. Sahmen, Z.A., IQ08,

1100°

57, I.

In-Tl

48 Tl,

Uncertain.

N". S. KurnakoffandN. A.

180°

Pushin,Z..4., 1907,63,430.

Ill

The metals are only partially miscible in the liquid state.

System.

Ag-Co
Ag-Cr
Ag-Fe
Ag-Mn

Remarks.

Immiscible at 1600°.
Horizontal at 1460°.
Immiscible at 1600°.
Horizontal at 1 160°.

Reference.

G. J. Petrenko, Z.A., 1907, 68, 212.
G. Hindrichs, Z,A., 1908, 69, 414.
G. J. Petrenko, Z.A., 1907, 53, 212.
G. Hindrichs, Z.A., 1908, 69, 414.

APPENDIX

403

System.

Remarks.

Refere

Ag-Ni

Al-Bi

Al-Cd

Al-K

Al-Na

Al-Pb

Al-Tl

As-Bi

Bi-Co
Bi-Cr
Bi-Fe

Bi-Si
Bi-Zn

Ca-Fe

Cd-Cr
Cd-Fe

Co-Pb
Co-Tl
Cr-Cu
Cr-Pb
Cr-Sn
Cr-Zn
Cu-Pb

Cu-Tl
Fe-Pb

Fe-Tl

K-Mg

Mg-Na

Mn-Pb

Ni-Pb

Ni-Tl

Pb-Si

Pb-Zn

Si-Tl
Tl-Zn

Horizontal at 1435°.

Nickel retains 4%

Ag in solid solution.

Very slight miscibility

at 650°.
Immiscible at 654°.
657°-
657°-
654°.
657°.
500°.

Immiscible at 1550°.
Immiscible at 400°.

Horizontal at 1464°.
1470°.
,, 1420°-

Slightly miscible.
Horizontal at 957°.

Horizontal at 960°.

Immiscible at 700°.
Horizontal at 638°.

"97°.

1338°-

Horizontal at 416°

G. J. Petrenko, Z.A., 1907, 83, 212.

A. G. C. Gwyer, Z.A., 1906, 49, 311.

A. G. C. Gwyer, Z.A., 1908, 67, 113.

D. P. Smith, Z.A., 1907, S6, 109.

C. H. Mathewson, Z.A., 1906, 48, 191.
A. G. C. Gwyer, Z.A., 1908, 57, 113.

F. Doerinckel, Z.A., 1906, 48, 185.
K. Friedrich and A. Leroux, M., 1908,

6, 158.
K. Lewkonja, Z.A., 1908, 69, 293.
R. S. Williams, Z.A., 1907, 65, i.

E. Isaac and G. Tammann, Z.A., 1907,
55, 58.

R. S. Williams, Z.A., 1907, 55, i.
W. Spring and L. Romanoff, Z.A., 1896,

13, 29.
C. Quasebart, M., 1906, 8, 28 ; O. P.

Watts, J. Amer. Chem.Soc, 1906, 28,

1152.

G. Hindrichs, Z.A., 1908, 59, 414.

E. Isaac and G. Tammann, Z.A ., 1907,
66, 58.

K." Lewkonja, Z.A., 1908, 53, 293.
K. Lewkonj^, Z.A., 1908, 59, 293.
G. Hindrichs, Z.A , 1908, 59, 414.
G. Hindrichs, Z.A., 190S, 69, 414.
G. Hindrichs, Z.A., 1908, 69, 414.
G. Hindrichs, Z.A., 1908, 59, 414.

C. T. Heycock and F. H. Neville, FAH.
Trans., 1897, 189a, 25.

F. Doerinckel, Z.A., 1906, 48, 185.
E. Isaac and G. Tammann, Z.A., 1907,

66, 58.
E. Isaac and G. Tammann, Z.A., 1907,
55, 58.

D. P. Smith, Z.A., 1907, 56, 109.

C. H. Mathewson, 2.^., 1906, 48, 191.
R. S. Williams, Z.A., 1907, 65, i.

G. Voss, Z.A., 1908, 57, 34.
G. Voss, Z.A., 1908, 67, 34.
S. Tamaru, Z.A., 1909, 61, 40.

W. Spring and L. Romanoff, Z.A., 1896,

13, 29.
S. Tamaru, Z.A., 1909, 61, 40.
A von Vegesack, Z.A., 1907, 52, 30.

404

ME TALLOGRA PHY

IV

Series containing a single compound.

^ Compound

System. formula.

Fropenies,

ReTerence.

Ag-Pt
Ag-Sb
Ag-Sn
Al-Fe

Al-Mg
Al-Sb
Al-Zn

Au-Na
Au-Sb

Au-Te

Bi-Mg

Bi-Te

Cd-Mg

Cd-Te

Hg-Te

Hg-Tl

K-Na
Mg-Pb .

Ag.Pt ?
AgjSb
AgjSn
AljFe

Al,Mg,

AlSb

AUZn^
(?)

AujNa
AuSbo

AuTej

BijMgj

BijTej

CdMg

CdTe

HgTe

Hg.Tl

KNa.
Mg.Pb

Formed from Pt and
liquid at 1185°. Forms
solid solutions with Ag.

Formed from Ag and
liquid at 560°. Solid
solutions.

Formed at 480° from a
solid sol. and liquid.
Transformation at 232°.

F.P. curve very imper-
fect. Found also by
analysis.

Max. F.P. at 4627".

Solid sols, with Al.
Max. F.P at? Formed

slowly in liquid.
Stable only between 443°

and 256°.

Max. F.P. at 989°.
No solid solutions.
Formed from Sb and

liquid at 460°. No solid

solutions.
Max. F.P. at 464°.

Max. F.P. at 716°.

solid solutions.
Max. F.P. at 573°.

No

Forms solid sols, with

both metals.
Max. F.P. at 1041°.

Max. F.P. at 605°,

Max. F.P. at 15°.

Formed from a and liquid

at 6-88°.
Max. F.P. at 551-3°.

No solid solutions.

F. Doerinckel, Z.A., 1907,
84. 333-

G. J. Petrenko, Z.A., 1906,
60, 133-

G. J. Petrenko, Z.A., 1907,
63, 200.

A. G. C.Gwyer, Z.A., 1908,

67, 113.
O. Brunck, Ber., 1901, 34,

2733-
G. Grube, Z.A., 1905, 45,

225.
G. Tammann, Z.A., 1905,

48, 53.
E. S. Shepherd, y./"., 1905,

9, 504; W. Rosenhain

and S. L. ArchbuU, Phil.

Trans., 1911, 211a, 315.
C. H. Mathewson, I.Z.M.,

1911, 1, 81.
R. Vogel, Z.A., 1906, 50,

145-

G. Pelliniand E. Quercigli,

A.A.L.,\^\o,\y\ 19,ii.445.
G. Grube, Z.A., 1906, 49,

72.
K. Monkemeyer, Z.A.,

1905, 46, 415.
G. Grube, Z.A., 1906, 49,

72.
M. Kobayashi, Z.A., 1910,

69, t.
G. Pellini, A.A.L., 1909,

[v] 18, ii. 211.
N. S. KurnakoffandN.A.

Pushin,Z./4.,l902, 30,86;

A. Sucheni, Zeitsch. Elek-

trochem., 1906, 12, 726.
N. S. Kurnakoff andN. A.

Fushin,Z./4.,i9O2,30, 109.
G. Grube, Z.A., 1905, 44,

1 17 ; N. S. Kurnakoff and

N. J. Stepanoff, ibid.,

1905, 46, 177,

APPENDIX

405

System.

Compound
formula.

Properties.

Reference.

Mg-Sb

Mg.Sb,

Max. F.P. at 950°.
solid solutions.

No

G. Grube, Z.A., 1906, 49,
72.

Mg-Si

Mg.Si

Max. F.P. at 1102°.
solid solutions.

No

R. Vogel, Z.A., 1909, 61,
46 ; P. Lebeau and P. Bos-
suet, R.M., 1909, 6, 273.

Mg-Sn

MgjSn

Max. F.P. at 795°.
solid solutions.

No

N. S. Kurnakofif and N. T.
Stepanoff,Z.^., 1905, 46,
177 ; G. Grube, ibid., 1905,
46, 76.

Mg-Zn

MgZn,

Max. F.P. at 595°.
solid solutions.

No

G. Grube, Z.A., igo6, 49,
72.

Pb-Te

rbTe

Max. F.P. at 917"'.

H.Fay and G. Gillson,.4w«-.
Chem. J., 1902, 27, 8i.

Pb-Tl

PbTlj?

Max. F.P. at 380°.

Solid

N. S. KurnakoffandN. A.

sols, with Pb and Tl.

Pushin, Z.A., 1907, 52,

430 ; K. Lewkonja, Z.A.,

tbid., 452.

Sb-Te

Sb.,Te,

Max. F.P. at 561°.

H. Fay and H. E. Ashley,
Amer. Chem. J., 1902, 27,
95-

Sb-Tl

SbTlj ?

Formed from solid solu-

R. S. Williams, Z.A., 1906,

tion at 187°.

50, 127.

Sn-Te

SnTe

Max. F.P. at 780°.

H.Fay, 7. Amer.Chem Soc,
1907, 29, 1265; M. Ko-
bayashi, Z.A., 1910, 69, i.

Te-Zn

TeZn

Max. F.P. at 1238-

5"-

M. Kobayashi, I.Z.M., 1912,
2,65.

Complex series in which two or more inter-metallic compounds are formed,
or in which the formation of immiscible liquid layers accompanies that
of compounds.

System.

Compounds.

Remarks.

Reference.

Ag-Al

AgsAl

Formed at 770° from solid solu-

G.J. Petrenko, Z.^.

tion and liquid.

190S, 46, 49-

Ag.Al

Formed at 719° from Ag^Al

and liquid.
Both form solid sols. AgjAl

forms and ;8 modifications.

AgAl (by E.M.F.) does not

exist.

4o6

METALLOGRAPHY
V — continued.

System,

Compounds.

Remarks.

Reference.

Ag-Cd

G. Bruni and E.

AgCd ?

Formed in solid at 200°.

Qnercigh, Z.A.,

Ag,Cd3

Formed from liquid at 633°.

1910, 68, 198; G.

AgCdj!

Formed from liquid at 568''.

J. Petrenko and
A. S. Fedoroff,
Z.A., 191 1, 70,
157; 71,215.

Ag-Mg

AgMg

Max. F.P. at 820°.

S. F. Schemtschu-

AgMg,

Formed at 492° from solid

schny, Z.A., 1906,

solution and liquid.

49, 400.

Ag forms solid sols, with 28%

Mg, and AgMg with 12 Ag

and 16 Mg.

Ag-Te

AgTe

Formed from liquid at 444°.

G. Pellini and E.

Ag,Te

Max. F.P. at 959°.

r

Quercigh, A.A.L.,
'910, [v] 19, ii,
415.

Ag-Zn

AgZn?

H. C. H. Carpenter

AgjZnj

Formed at 66^° from ;8 and

and W. Whiteley,

liquid.

I.Z.M., 1912, 3,

Ag^Ziis

Formed at 636° from Ag2Zn3
and liquid.

145-

Al-Au

AljAu

-Max. F.P. at 1060°.

C. T. Heycock and

AlAu

Formed at 625° from Al^Au

F.H. Neville, /'/5;7.

and liquid.

7>a«j-., 1900, 194a,

AlAuj

Max. F.P. at 624°.

201.

AljAu,

Formed at 575° from AlAu^
and liquid.

AlAu J

Formed at 545° from Au and
liquid.

Al-Ca

AljCa

Crystallizes at 692°.

L. Doriski, Z.A.,

Horizontal at 692° from 1 1 to

1908, 67, 185.

34 atom. % Ca.

Al-Co

A1,,C03?

Formed at 940° from AI5C02

A. G. C, Gwyer,

and liquid.

Z.A., 1908, 57,

AI5C02

Formed at 1165° from AlCo
and liquid.

113-

AlCo

Max. F.P. at 1628°.

The series includes solid
sols., and undergoes some
transformations in the solid
state.

Al-Cr

AlCr3 ?

Very' high m.p. Found by

G. Hindrichs, Z.A.,

extrapolation. Two liquid

1908, 59, 414.

layers over part of range.

APPENDIX

407

System.

Compounds.

Remarks.

Reference.

Al-Cu

Al^Cu

Formed at 588° from AlCu

H. C. H. Carpenter

and liquid.

and C. A. Edwards,

AICu

Formed at 625° from solid

Proc. Inst. Mech.

solution and liquid.

■Eiig., 1907, 57;

AICU3

Max. F.P. at 1050°.

B. E. Curry, J.F.,

AlCuj?

A coinplex system, in which
the solid sols, vary greatly in
composition with the tem-
perature.

1907, 11, 425.

Al-Mn

AljMn?

Max. F.P. at 1279°.

G. Hindrichs, Z.A..

AlMns

Two liquid layers also formed.

1908, 59, 414.

Al-Ni

AljNi

Formed at 835° from AljNi

A. G. C. Gwyer,

and liquid.

Z.A., 1908, 87,

Al,Ni

Formed at 1130° from AlNi
and liquid.

"3-

AlNi •

Max. F.P. at 1629°; forms
solid solutions with Ni.

As-Cu

AsjCuj

Formed at 711° from AsCu,

K. Friedrich, M.,

and liquid.

1908, 5, 529.

AsCuj

Max. F.P. at 830°

Au-Cd

Au^Cdj

Formed at 625° from Au and

R. Vogel, Z.A.,

liquid.

1906, 48, 333.

AuCd,

Formed at 495° from Au^Cdj

and liquid.
Solid solutions formed.

Au-Mg

AuMg

Mat. F.P. at 1150°.

R. Vogel, Z.A.,

AuMg2

1788°
,, ,, 700 .

1909, 63, 169 ; G.

AujMgs

Formed at 796° from AuMgs

G. Urazoif, Z.A.,

and liquid.

1909, 64, 375 ; G.

AuMgj

Max. F.P. at 818°.

G. Urazoff and R.

Solid solutions formed.

Vogel, Z.A., 1910,
67, 442.

Au-Pb

AujPb

Formed at 418° from Au and

R. Vogel, Z.A.,

liquid.

190S, 45, u.

AuPbj

Formed at 254° from AujPb

and liquid.
AuPbj has a transformation

point at 211°. No solid sols.

Au-Sn

AuSn

Max. F.P. at 418°.

R. Vog^l, Z.A.,

AuSn,

Formed at 308° from AuSn
and liquid.

1905, 46, 60.

AuSn^

Formed at 252° from AiiSn^
and liquid.

Au-Zn

AuZn

Max. F.P. at 744°.

R. Vogel, Z.A.,

AujZns

Formed at 651° from solid
solution and liquid.

1906, 48, 319.

AuZn, ?

Formed at 490° from solid
solution and liquid.

ioi

MS.TALLOGRAPHV

V — continued.

System.

Compounds.

Remarks.

Reference.

Bi-K

Bi-Na
Bi-Ni
Bi-Tl
Ca-Cd

Cd-Cu

Cd-K
Cd-Na

Cd-Sb

Bi,K
Bi,K„ ?

BijK,

BiK,

BiNa

BiNaj

BijNi

BiNi

Bi,Tl3
BiTl,

CaaCdj

CaCd
CaCdj

CaZn ?

CaZn,

CaZiiij
CdjCuj

CdCuj

Cd,K?
Cd,iK ?

CdjNa

CdjNa

CdjSbj
CdSb

Max. F.P. at 540°.

Formed at 373° from BijKs

and liquid.
Formed at 420° from BiKj

and liquid.
Max. F.P. at 671°.
No solid solutions.
Formed at 445° from BiNa,

and liquid.
Max. F. P. at 775°.
No solid solutions.
Formed at 469° from BiNi and

liquid.
Formed at 655° from Ni and

liquid.
Max. F.P. at 227°.
The curve is unsatisfactory,

and there is probably a third

compound.
Formed at 510° from CaCd

and liquid.
Crystallizes at 685°.
Formed at 615° from CaCd

and liquid.
Horizontal at 685° from 28 to

83 atom. % Ca.
Formed at 431° from Ca.Znj

and liquid.
Max. F.P. at 688°.
Formed at 680° from CaZn^

and liquid.
Max. F.P. at 717°.
Max. F.P. at 565°. Forms

solid solutions.
Formed at 552° from Cu and

liquid.
Very doubtful.
Max. F.P. at 473°.
Horizontal at 468°-
Max. F.P. at 363°.

., 385°.
Horizontal at 330° from 58 to
70 atomic % Na.

Formed at 409° from CdSb

and liquid.
Max. F.P. at 455°
The latter is often suppressed

by undercooling.

D. P. Smith, Z.A.,
1907, 56, 109.

C. H. Mathewson,
Z.A., 1906,50,171.

G. Voss, Z.A., 1908,
87, 34.

M.Chikashige, ^'.v^.,
ig»6, 61, 328.

L. Donski, Z.A.,
1908, 57, 185.

L. Donski, Z.A.,
1908, 67, 185.

R. Sahmen, Z.A.,
1906, 49, 301.

D. P. Smith, Z.A.,
1907, 56, 109.

C. H. Mathewson,
Z.A., 1906,50,171 ;
N. S. Kurnakoff
and A. N. Kusnet-
zoff, ibid., 1907, 62,
173-

W. Treitschke, Z.A.,
1906, 60, 217; N.
S. Kurnakoff and
N.A.Konstantinoff,
ibid,, 1908, 68, I.

APPENDIX

409

System.

Co-Sb

Co-Si

Co-Sn

Cr-Sb

Cs-IIg

Cu-Mg

Cu-Sb

Cu-Si

Cu-Sn

Cu-Te

Compounds.

CoSb
CoSbj

CojSi
CojSij ?
CoSi
CoSi,

CoSij
CojSn
CoSn

CrSb

CrSb.

Cs,Hg ?

CsHg?

CsHg2

CsIIg.

CsHg,

CsHgio ?

CujMg
CuMgj

Cu,Sb
Cu,Sb

Cuj.Si

Cji,Sn ?
CujSn

CuSn ?

Cu.Te

Remarks.

Reference.

-a

Max. F.P. at 1191°.

Formed at 898° from CoSb

and liquid.
Max. F.P. at 1327°.
Formed slowly at 1180-1215°.
Max. F.P. at 1395°.
Formed at 1277° from CoSi

and liquid.
Max. F.P. at 1310°.
Max. F.P. at 1171°.
Formed at 927° from Co and

liquid.
CoSn lias a transition- point at

515° ; both o and ;8 forms are

non-magnetic.
Max. F.P. at 1112°. Forms

solid solutions with Or.
Formed at 675° from CrSb

and liquid.

Max. F. P. at 2oS°.
„ „ 163-5°.

Formed from CsHg, and liquid

at 13°.
Max. F.P. at 800°.

» S7S°-
No solid solutions.

Max. F.P. at 680°.

Formed from CujSb and liquid

at 580°.
Complex transformations in

solid state.
Max. F.P. at 862°.
The remainder of the system

is doubtful.

The only compound estab-
lished with certainty.

Formed at 855° from Cu and

liquid.
Two polymorphic changes at

384° and 355°.

K. Lewkonja, Z.A.,
1908, 59, 293.

K. Lewkonja, Z.A,,
1908, 59, 293.

S. F, Schemtschu-
schny and S. W.
Belynsky, Z.A.,
igo8, 59, 364 ; K.
Lewkonja, ibid.,

293-
R. S. Williams, Z.A.,
1907, 55, I.

N. S. Kurnakoff and
G. U. Schukowsky,
Z.A., 1907,52,416.

R. Sahmen, Z.A.^
1908, 67, I ; G. G.
Urazoff, y. Riiss.
Phys. Chem. Soc,
1907, 39, 1566.

A. Baikoflf, J. Puss,
Phys. Chem. Soc,
1904, 36, III.

E. Rudolii, Z.A.,
1907, 63, 216; P.
Lebeau, Compt.
rend., 1906, 142,
154.

C. T. Heycock and
K. H.Neville, /V«7.
7Va«J., 1903, 202a,
I ; F. Giolitti and
G. Tavanti, Gaz-
zelta, 1908, 38, ii.
209.

M.Chikashige,2'./^.,
1907. 54, 50.

4IO

METALLOGRAPHY

V — continued.

System.

Compounds.

Remarks.

Reference.

CuiTej

Formed at 620° from 7CujTe

and liquid.
Polymorphic change at 620°.

Two liquid layers at 1030°

from 68 to 90 atomic %

Cu.

Cu-Zn

CuzZn ?

Formed al 905° from solid

E.S.Shepherd,y./'.,

and liquid.

1904,8,421 ; V.E.

CUjZhs

Formed at 830° from /3 solid

Tafel, M., 1908, 6,

and liquid.

343-

Other compounds may exist,

but their formulse are un-

certain.

Fe-Sb

Fe.Sb,

Max. F.P. at 1016°. Solid

N. S. Kurnakoffand

solutions with Fe and Sb.

N. Konstantinoff,

FeSb,

Formed at 730° from Fe^Sbj
and liquid.

Z.A., 1908, 58, I,

Fe-Si

Fe^Si

Formed at 1250° from solid

W. Guertler and G.

solution and liquid.

Tammann, Z.A.,

FeSi

Max. F.P. at 1443°.

1905, 47, 163.

Fe-Sn

?

Horizontal at 1140°. Iron

E. Isaac and G.

retains some Sn in solid solu-

Tammann, Z.A.,

tion. The evidence for a com-

1907, 63, 281.

pound depends on thermal

arrests at 893°, 780°, and

496°. The formula is quite

undetermined.

Hg-K

HgK

Formed from HgjK and liquid

E. Janecke, Z.P.,

at 178°.

1907, 53, 245.

Hg.K

Max. F.P. at 270°.

Hg3K?

Formed from Hg^K and liquid
at 204°.

Hg,K, ?

Formed from HgjK and liquid
, at 173°.

Hg.K

Formed from HgjKj and liquid
at 70°.

Hg-Na

Hg,Na

Formed from HgjNa and

A. Schiiller, Z.A.,

liquid at 159°.

1904, 40, 385 ; E.

Hg.Na

Max. F.P. at 360°.

Vanstone, Trans.

Hgi,Na,2

Formed from IlgzNa and

Faraday Soc. , 1 9 1 1 ,

(or HgjNa,)?

liquid at 227°.

7, 42. •

HgNa

Formed from HguNaij and
liquid at 219°.

Hg.Na,

Formed from HgNa and
liquid at 123°.

Hg.Na, ?

Formed from HgjNaj and
liquid at 62°.

HgNaj

Formed from Hg._,Na3 and
liquid at 34°-

APPENDIX

411

System.

!^ompounds.

1
Remarks.

Reference.

Hg-Rb

HgeRb

Max. F.P. at 136-5°.

N. S. Kurnakoff and

Possibly a second compound .

G. U. Schukowsky,
y. Russ. Phys.
Client. Soc, 1906,
88, 1216.

K-Pb

Horizontal at 568° from 35 to

D. P. Smith, Z.A.,

75 atomic % K.

1907, 66, 109.

K,Pb ?

Formed from unknown solid
at 375°.

KPb,?

Formed from unknown solid
at 337°-

KPb<?

Formed from KPb, and liquid

at 295°. _
Very uncertam.

K-Sn

KjSn ?

Formed from KSn and liquid

D. P. Smith, Z.A.,

at S35°-

1907, 66, 109.

KSn?

Formed from an unknown
compound at 670°.

KSiij

Formed from an unknown
compound at 600°.

KSn,

Formed from KSnj and liquid
at 413°.

These formulae are very doubt-
ful.

Formed from KTl and liquid

K-Tl

KjTl ?

N. S. Kurnakoff and

at 240°.

N.A.Pushin.2.^.,

KTl

Max. F.P. at 335°.

1902, 80, 86.

K-Zn

KZn„ ?

Very doubtful.

D. P. Smith, Z.A.,

Horizontal at 585°. Com-

1907, 86, 109.

pound supposed to undergo

transformation near 500°

Li-Sn

LijSn

Max. F.P. at 680°.

G. Masing and G.

LisSn^

„ » 465°.

Tammann, Z.A.,

LijSns

Formed from liquid at 320.°

1910, 67, 183.

Mg-Ni

Mg,Ni

Formed at 76S" from MgNij

G. Voss, Z.A., 1908,

and liquid.

87, 34-

MgNi^

Very flat max. F.P. at 1145°.

Mg-Tl

Mg,Tl,?

Max. F.P. at 415°.

G. Grube, Z.A.

Mg,Tl

Pormed at 393° from MggTlj
and liquid.

1905, 46, 76.

Mg^Tl,

Formed at 355° from MgjTl
and liquid.

Mn-Sb

Mn^Sb

Max. F.P. at 920°. Solid

R.S.Williams,2./i.,

solutions with Mn and Sb.

1907, 86, I.

MnjSbj

Formed at 850° from solid solu-
tion (MnjjSb) and liquid.
Solid solutions with Sb.

Mn-Si

Mn^Si

Max. F.P. at 1316°.

F. Doerinckei, Z.A.,

MnSi

„ „ 1280°.

1906, 60, 117.

Mn-Sn

MnjSn

Formed at 989°.

R.S.Williams,^:.^.,

Mn.Sn

„ 898°.

1907, 66, I.

412

METALLOGRAPHY

V — continued.

System.

Compounds.

Remarks.

Reference.

MnSn

Formed at 541°.

Na-Pb

Na^Pb

Max. F.P. at 386°.

C. H. Mathewson,

NajPb

„ „ 405°.

Z.A., 11)06, 50, 171,

NaPb

„ „ 367°.

Na,Pb.

" '■ 319°

Na-Sb

Na,Sb

Max. F.P. at 856°.

C. H. Mathewson,

NaSb

,■ „ 465°.

Z.A.,igo6,6Q, 171.

Na-Sn

Na^Sn

Formed from NajSn and liquid

C. H. Mathewson,

at 405°.

Z.^., 1906,60, 171.

Na^Sn

Max. F.P. at 477"-

NajSns

Formed from NaSn and liquid
at 473°-

NaSn

Max. F.P. at 576°.

NaSns

Formed from NaSn and liquid
at 405°.

Na-Te

Na„Te

Max. F.P.

G. Pellini and E.

Na3Te,

Quercigh, A.A.L.,

XajTe,

i9io,[v]19,ii.35o.

Na-Tl

Na.Tl,?

Formed from Na^TI and liquid

N. S. Kurnakoffand

at 78°.

N. A.Pushin,Z.^.,

Na^Tl?

Formed from NaTl and liquid
at 159°.

1902, 30, 86.

NaTl

Max. F.P. at 306°.

Na-Zn

NaZni, ?

Doubtful. Horizontal at 557°.

C. H. Mathewson,
Z./4., 1906,48, 191.

Ni-Sb

NijSb

Formed at 670° from solid sol .

K. Lossew, Z.A.,

Ni^Sb,

Max. F.P. at 1170°. Solid

1906, 49, 58; y.

solutions with Ni.

Muss. Phys. C/tem.

NiSb

Max. F.P. at 1107°. Solid
solutions with Ni.

Soc ,1^X1, iZ,zi 3.

NijSb,

Formed at 618° from NiSb
and liquid.

Ni-Si

NijSi ?

Formed near 1125° from solid

W. Guertler and G.

solution.

Tammann, Z,A.,

Ni.Si

Max. F.P. at 1310°.

1906, 49, 93.

NijSi,?

Formed near 830° from solid
solution.

NiSi

Max. F.P. at 1000°.

NijSij

Formed at 985° from Si and

liquid.
The diagram is complex and

imperfectly known.

Ni-Sn

Ni,Sn

Formed at 855° from solid.

G. Voss, 2r.^.,i9o8,

NijSn

„ 1162° from NijSn,

57, 34 ; L. Guillet,

and liquid.

Bull, Soc. chim. ,

NijSnj

Formed at 1263°.

l<^^, [iv.] 1, 775.

Ni-Zn

NiZn,

Max. F.P. at 876°.

V.E.Tafel,i/.,i9o8,

NiZn

Formed from liquid at 1035".

8, 413 ; G. Voss,
Z.A., 1908, 57, 34.

APPENDIX

412

System.

Compounds.

Remarks.

Reference.

Pb-rd

PbjPd

Max. F.P. at 457°

R. Ruer, Z.A., 1907,

PbPd

Formed at 595° from solid

52, 345; N. A.

solution and liquid.

Pushin and N.

PbPdj

Formed at 830° from PbPds

Paschsky, J. Russ.

and liquid.

Phys. Chem. Soc,

PbPdj

Max. F.P. at 1220°

1908, 40, 826.

Pb-Pt

PbjPt

Formed at 356° from PbPt

F. Doerinckel, Z.A.,

and liquid.

1907, 64, 333 ; N.

PbPt

Formed at 787" from an un-

A. Pushin and P.N.

known compound and liquid.

Laschtschenko,
ibid., 1909, 62, 34.

Pd-Sb

PdjSb

Max. F.P. at 1220°.

W. Sander, Z.A.,

PdjSb,

Formed at 839°.

1912, 76, 97.

PdSb

Formed at 805°.

PdSbj

Max. F.P. at 680°.

Pt-Sb

PtjSbj

Formed in solid at 644°.

K. Friedrich and A.

PtSb

Formed from PtSbj and liquid

Leroux, M., 1909,

at 1045°.

6,1.

PtSbj

Max. F.P. at 1226°.

Pt-Sn

PtSiij

Formed at 540° from oPtjSnj

N. Podkopaeff, J.

and liquid.

Russ. Phys. Chem.

PtjSnj

Formed at 850" from PtSn

^■ai-., 1908,40, 249;

and liquid . Polymorphic

F.Doerinckel,Z.^.,

change at 740°.

1907, 64, 333.

PtSn

Max. F.P. at about 1300°.

PtsSn

Formed at about 1400° from
Pt and liquid.

Sb-Sn

SbSn

Four series of solid solutions,

J. E. Stead, J. Soc.

SbjSnj

with breaks at 7'S and 5°

Chem. hid., 1897,

atomic % Sb.

16, 200 ; W. Rein-
ders, Z.A., 1900,
25,113; R.S.Wil-
liams, ibid., 1907,
56, I.

Sb-Zn

SbZn

Formed at 535° from SbjZnj

S. F. Schemtschu-

and liquid.

schny, Z.A., 1906,

SbjZnj

Max. F.P. at 566°. Poly-

49,384 ; K. Monke-

morphic change at about 355°.

meyer, ibid., 1905,

Formation of SbZn easily sup-

43, 182.

pressed by undercooling.

AH

METALLOGRAPHY

VI

Systems which have not been completely investigated. In some instances
the freezing-point determinations have been limited to a certain range
of composition by the volatility of one of the components under
atmospheric pressure.

System. Compounds.

Ag-As

Ag-Hg
As-Co

AgsHg,

AsCo
AsjCoj

AsCOj

AsoCo,

As-Fe

AsFe

AsjFej

AsFe^

As-Mn

AsMn,

AsMn

AsjMhs ?

As-Ni

AsNi

As„Ni3 ?

As^Nis

As-Pb

?

As-Pt

As^Ptj

As-Sb

As-Sn

AsjSn^

AsSn

As-Zn

AsjZnj ?

Bi-Ca

?

Remarks.

A compound AgjAs is possible.

Solid solutions are formed.

Max. F.P. near ii8o°.
Formed at 1010° from AsCo

and liquid.
Formed at 960° from AsjCoj

and liquid.
Formed at 900° from AsCoz

and liquid.
A complicated system, with

many transformations in the

solid state.
Max. F.P. at 1030°.
Formed at 800° from solid.
Max. F.P. at 919°.
Max. F.P. at 1022°.

.. .- 9SS°-
Formed from solid.
Max. F.P. at 968°.
Formed near 800°.
Max. F.P. at 998°.
No compound observed with

certainty.
Max. F.P. near 1450° ; found

by slight extrapolation.

Solid solutions from o to 50
atom % As.

Formed at 578°.
Max. F.P. at 585°.

One compound of uncertain
composition.

One compound of unknown
composition, formed at 500°.

Reference.

K. Friedrich and A.

Leroux, M., 1906,

3, 192.
A. Ogg, Z.P., 1898,

27, 285.
K. Friedrich, A/.,

1908, 5, 150.

K. Friedrich, /)/.,

1907, 4, 129.

P. Schoen, M., 191 1,
8. 739-

K. Friedrich and F.

Bennigson, 1907,4,

200.
K. Friedrich, M.,

1906, 3, 41.
K. Friedrich and A.

Leroux, M., 1908,

5, 148.
N. Parravano and P.

de Cesaris, I.Z.M.,

1912, 2, 70.
N. Parravano and P.

de Cesaris, /.Z.jT/.,

1912, 2, I.
K. Friedrich and A.

Leroux, M., 1906,

3. 477.
L. Doiiski, Z.A.,

1908, 57, 185.

APPENDIX

415

System.

Compounds.

Remarks.

Reference.

Ca-Cu

?

Only a very small range in-

L. Doiiski, Z.A.,

vestigated.

19 8, S7, 185.

Ca-Pb

CaPbj

Max. F.P. at 650°.

L. Doiiski, Z.A.,
1908, 57, 185.

Ca-Sn

CaSnj

Max. F.P. at 624°.

L. Donski, Z.A.,
1908, 67, 185.

Ca-Sb

No compound found within

L. Doriski, Z.A.,

the range investigated.

1908, 57, 185.

Ca-Si

CaSia

Formed at about 990°. Course

S. Tamaru, Z.A.,

of F.P. curve doubtful.

1909, 63, 81.

Ca-Tl

CaTl3

Formed at 524°-

L. Doriski, Z.A.,

CaTl ?

1908, 67, 185.

Cd-Ni

Cd,Ni

Formed at 501° from an un-

G. Voss, Z.A., 1908,

known compound and liquid.

57, 34.

Co-P

C02P

Max. F.P. at 1386°.

S. F. Schemlschu-
schny and J. Sche-
peleff, Z.A., 1909,
64,245-

Co-Zn

CoZnj ?

Doubtful.

K. Lewkonja, Z.A.,
1908, 69, 293.

Cr-Fe

?

F. P. curve very irregular, and

W. Treitschke and

formula of compound not

G. Tammann,2./i.,

determined.

1907, 65, 402.

Cu-P

Cr,P

Max. F.P. near 1022°. A

E. Heyn and 0.

higher phosphide, perhaps,

Bauer, Z.A., 1907,

also formed.

52, 129; A. K.
Huntington and C.
H. Desch, Trans.
Faraday Soc, 1908,

4, SI-

Lautsch and G. Tam-

Fe-Mo

?

F.P. curve irregular. Formula

of compound not determined.

mann, Z.A., 1907,
56, 386.

Fe-Ni

FeNi,, ?

Formula doubtful. F.P. curve

W. Guertler and G.

continuous.

Tammann, Z.A.,
190S, 46, 20s ; R.
Ruer and E. Schiiz,
M., 1910, 7, 415.

Fe-P

Fe^P

Formed at 1130°.

N. A. KonstantinoflT,

FejP

Max. F.P. at 1350°.

Z.A., 1909, 66,
209.

Fe-Pt

Isomorphous, solid solutions

E. Isaac and G. Tam-

breaking up at a lower tem-

mann, Z.A., 1907,

perature.

56, 63.

Fe-W

?

F.P. curve irregular. Formula

H. Harkort, M.,

of compound not determined.

1907, 4, 617, 639,

673-
A. von Vegesack,

Fe-Zn

FeZrij ?

Formed at 777°-

FeZn, ?

„ 662° from FeZua
and liquid.

Z.A., 1907, 52, 30.

4[6

METALLOGRAPHY
VI — continued.

System.

Compounds.

Remarks.

Reference.

Mn-P

Mn^Pj

Max. F.P. at 1390^.

S. F. Schemtschu-

MnP

The F.P. curve rises from a

schny and N. N.

eutectic point, indicating a

Efremoff, J. RusS.

probable max. near 1180°;

Phys. Chem. Soc,

but the system has only been

1907, 39, 777.

inyestigated as far as 47

atomic % P.

Ni-P

Ni^P

Formed at 965° from ;8Ni5P2

N. A. Konstantinoff,

and liquid,

Z.A., 1908, 60,

Ni.P.

Max. F.P. at 1185° Poly-
morphic change at 1025°.

405.

Ni,P

Max. F.P. at 1112°. At
least one more compound
must exist.

VII

Ternary Systems

The freezing-point or liquidus surfaces of most of the following systems
have been determined accurately. In most cases the solidus has also
been determined, whilst the study of the exact nature of the trans-
formations in tlie solid state is less complete.

System.

Ag-Au-Cu
Ag-Cu-Pb

Ag-Pb-Sn
Ag-Pb-Zn

Al-Cu-Sn

Al-Cu-Zn
Bi-Cd-Pb

Remarks.

Solid solutions with a gap. No
compound or ternary eutectic.

No ternary compound. One
ternary eutectic, and a region
of limited miscibility.

No ternary eutectic.

No ternary eutectic. The lead
has little effect on the AgZn
system .

No ternary compound. Com-
plex changes in solid solu-
tion.

Partially investigated.

Ternary eutectic at 9r5°

Reference.

E. Janecke, M., 1911, 8, 597.

K. Friedrich and A. Leroux,
M., 1907, 4, 293.

N. Parravano, I.Z.M., 191 1,

1,89.
R. Kremann and F. Ilof-

meier, Monatsh., 19 11, 32,

563-
J. H. Andrew and C. A.

Edwards, Proc. Roy. Soc,

1909, 82a, 568 ; y. Inst.

Metals, 19 10, 2, 209.
H. C. H. Carpenter and C.

A. Ed wards, /.Z.il/., 1912,2,

209.
W. E. Barlow, y. Amer.

Chem. Soc, 1910, 32, 1390.

APPENDIX

417

System.

Bi-Cd-Sn
Bi-Cu-Sb

Bi-Pb-Sn

Ccl-Cu-Sb
Cd-Hg-Na
Cd-Hg-Pb
Cd-Mg-Zn

Cd-Pb-Sn

Cu-Fe-Mn

Cu-Fe-Ni
Cu-Mn-Ni

Cu-Pb-Sn

Cu-Ni-Zn

Hg-K-Na

Mg-Pb-Sn

Pb-Sb-Sn
Pb-Sn-Zn

Remarks.

No ternary compound. One

ternary eatectic.
No ternary compound. Two

liquid phases.

No ternary compound. One
ternary eutectic.

Stable and metastable systems.

? CdHgNa ; max. F.P. at 325°.

No ternary compound.

Ternary eutectic at 250°.

No ternary compound. One

ternary eutectic.
Solid solutions with gap.

Solid solutions with gap.
Solid solutions.

Partially investigated.

No ternary compound. Com-
plex changes in solid solution.
? HgjKNa J max. F.P. at 188°.

No ternary compound.

No ternary eutectic.
Ternary eutectic at 177°-

Reference.

A. Stoffel, Z.A., 1907,. 63,
139-

N. Parravano and E. Viviani,
A.A.L., 1910, [vj 19, ii.
lo68.

G. Charpy, Etude des Al-
liages, 201 ; E. S. Shep-
herd, y.P., igo2, 6, 519.

A. P. Schleicher, I.Z.M.,

I9I2, 3, I02.

E. Janecke, Z.P., 1906, 57,
507-

E. Janecke, Z.P., 1907, 60,
399-

G. Bruni, G. Sandonnini, and

E. Quercigh, Z.A., 1910,

68, 73; G. Bruni and G.

Sandonnini, Z.A., 1912, 78,

273-
A. Stoffel, Z.A., 1907, 63,

129.
N. Parravano, Gazzetta, 1912,

42, ii. 513.
R. Vogel, Z.A, 1910, 67, i.
N. Parravano, Gazzetta, 191 2,

42, ii. 385.

F. Giolitti and M. Maranto-
nio, Gazzetta, 1910, 40, i. 51.

V. E. Tafel, M., 1908, 5,

413-
E. Janecke, Z.P., 1906, 67,

S07.
A. von Vegesack, Z.A., 1907,

84, 367.
R. Loebe, M., 191 1, 8, 7, 33.
M. Levi-Malvano and O.

Ceccarelli, Gazzetta, 191 1,

41, ii. 269,

T.P.C-

Z E

4i8

METALLOGRAPHY

VIIl

Quaternary Systems.

The tctrahedral space-model of the following system has been deter-
mined so far as concerns the liquidus surfaces.

System.

Bi-Cd-Pb- Quaternary eutectic at 70°.
Sn

Cu-Fe-Mn-

Ni

Solid solutions.

Reference.

N. Parravano and G. .Siro-
vich, Gaezeita, 1912, 42, i.
630.

N. Parravano, ibid., igi2, 42,
ii. 589-

SUBJECT INDEX

Aeolic, 59

AUotropy of iron, 36a
Alloys, nature of, 5

, preparation of, io6, ijg

Alternating stress, ^5°
Alumina, for polishing, 140
Aluminium, alloys of, 396
Amalgams, E.M.F. of, 332
Amorphous modifications of metals,

355
Annealing, 47, 225
Atmosphere for fusion, 102
Atomic concentration, 20

fall, 21, 329

percentages, 20

Austenite, 364, 373

B

Bearing-metals, 396
Beck illuminator, 156
Bell-metal, 391
Boiling-points of metals, 108
Brasquing, of crucibles, 105
Brass, 183, 387

, naval, 388

Brinell's test, 245
Britannia metal, 396
Bronze, 390

, aluminium, 392

, manganese, 394

, plastic, 397

Burnt metals, 228

Camera, photographic, 170
Carbide carbon, 362
Carbides, 193
Castings, weakness in, 178
Cast surfaces, 152

419

Cementation, no, 217
Cementite, 365, 371
Coercive force, 275
Coinage, 391, 395
Cold junction, 113
Colloidal suspensions, 375
Colonies, 185
Colour filters, 169

photography, 173

Colours, temper-, 229
Component, definition of, 23
Compounds, formation of, in solid

solutions, 62

, inter-metallic, 32, 64, 337

Conduction, theory of, 251
Conductivity, electrical, 251

, thermal, 262

Conglomerates, 23

Conjugate alloys, 88

Constantan, 261, 394

Constituents, micrographic, 26

Cooling curve, form of, 13

: curves, methods of plotting,

123 ff-
Copper, alloys of, 387
Cores, 182, 387
Corrosion, 288
Critical curve, 89

solution temperature, 82

surface, 89

Crucibles, refractory, 104
Cryohydrates, 22
Crystal grains, 177
Crystallites, 17s
Crystallization, 175 ff.

interval, 46

Crystal skeletons, 175

Delta metal, 389

Density, 232

Deoxidation, 107

Derived differential curves, 126

420

SUBJECT INDEX

Devitrification, 199
Diamantine, 140
Difference methods, 116
Diffusion, in solids, 46, 216 ff.
Diffusivity, 217, 334
Dilatometers, 239
Direct-rate curves, 125
Dispersion, ultramicroscopic, 225
Dissociation on fusion, 336

below melting-point, 36

Duralumin, 396

E

Edges, protection of, 145
Elastic limit, 341

reaction, 246

Electrolytic potential, 276, 332

of hardened metals, 359

Emery papers, 136
Enclosures, mechanical, 195
Equivalence, coefficient of, 191
Equilibrium diagram, conjtruction of,

299 ff.
Etching, electrolytic, 148
Etching-pits, 179
Etching reagents, 145
Eutectic alloys, 21

, structure of, 183

point, 16

• ■, ternary, 69

times, 18, 307

Eutectoid, 59, 366
Exfoliation, 293
Expansibility, thermal, 238
Eyepieces, microscopical, 165

False equilibrium, 33

Fatigue, 350

Ferrite, 369

Ferronite, 372

Ferroxyl reagent, 289

Fers de lance, 374

Fictitious values, in ternary alloys,

191
Films, properties of, 358
Flashing, of gold beads, 198
Flattening, of maxima, 34, 336
Flow of metals, 343
Fractures, 2, 353
Freedom, degrees of, 24
Freezing mixtures, 21
Freezing-point curve, 6, 12, 20

, depression of, 15, 329

surface, 67

Freezing-points, standard, 114

Furnaces, electric, carbon tube, gS

, granular resistance, 99

, Heraeus, 96

, iridium tube, 98

, Nernst tube, 99

— -■ gas, 94

Fusible metals, 69

Fusion, change of volume on, 242

, latent heat of, 329

Galvanizing, 290
Galvanometers, 114, 118, 213
German silver, 394
Glaciers, 226, 344
Glass, electrolysis of, 220
Gliding-planes, 347
Gold, alloys of, 395
Graphite, 370
Grinding sections, 136
Growth of crystals, 176, 226
Gun-metal, 390

H

Hardening carbon , 362

Hardness, 243

Heating curves, 304, 316

stage, 149

Heat of fusion, 13, 329

solution, 296

Heat-tinting, 148
Heusler's alloys, 272
Hydrates, melting of, 36
Hypertectic point, 200
Hysteresis, magnetic, 274

Ice, plasticity of, 344
Ideal alloys, 89

curve, 330

lUuminants, 166
Illumination, oblique, 153

, vertical, 156

Immiscible liquids, 81
Indentation test, 245
Industrial alloys, 386 ff.
Ingots, brittleness of, 178
Inter-metallic compounds, 32, 64, 337

• , conductivity of, 256

, crystallographio characters

of, 188

, dissociation of, 336

, hardness of, 250

Inverse-rate curves, 125

SUBJECT INDEX

421

Iron, allotropy of, 362

carbides, possible, 379

carbon system, 361 fif.

, cast-, structure of, 2, 367, 377

, technical varieties of, 376

Isomorpliism, 44
Isothermal lines, 70

I

Japanese swords, metastable state of,

215

Journals, metallographic, 9

K

Kamacite, 383
Kish, 361

Labile region, 200

Lamps, for microscope, 166

Latent heat, 13, 124, 329

Lead, alloys of, 395

Liquid solubility, 81, 312

Liquidus, 46, 302

Literature of metallography, 9

M

Magnalium, 396

Magnetism, 265

Magnetometer, 266

Manganic, 394

Martensite, 374

Maximum freezing-point, 33

M61jer burner, 95

Melting, change of volume on, 242

under pressure, 357

Melting-point, 28

• , influence of size of grain

on, 29
Metals, crystallographic characters

of, 181

, purity of, 130

Metastable limit, 200
Meteorites, 3, 383
Micrographia, 2
Micrographic constituents, 26
Microsclerometer, 244
Microscope, construction of, 154
Micro-sections, 134
Minimum freezing-point, 50
Miscibilily, partial, 53, 81

Mixed crystals, 7, 44
Molecular weight of metals, 329 ff.
Mounting device, 160
Muntz's metal, 389

N

Naval brass, 389

Nichrome, 98

Non-metals, in alloys, 131, 193

Objectives, microscopical, 163

Optical bench, 168

Organic substances, mixtures of, 200,

203, 206
Osmondite, 376 .
Oxides, 194
Oxygen, removal of, from alloys, 107

Partition-coefficient, 90

Patina, on bronze, 391

Pattinson's process, 31

Pearlite, 61, 185, 366, 371

Percussion figures, 349

Pewter, 395

Phase, definition of, 23

— — rule, apparent exceptions to, 72

Phases, doctrine of, 6, 23

, limits of, 319

Phosphor-bronze, 393
Photography, 172

in colours, 173

Planimetric analysis, 324
Plasticity, 342
Platinoid, 261, 395
Plessite, 384
Polarization, 277
Polish-attack, 150
Polishing machine, 137, 142

powder, 140

■ sections, 139

Polish, nature of, 140, 353
Polymorphism, 58, 314

, effect of, on E.M.F., 285

Potential, electrolytic, 276

, , effect of strain on, 286

Potentiometer, 115

Powders, polishing, 140

Prism illuminator, 157

Pyrometers, electrical resistance, 12a

, optical, 120

, radiation, 122

— — , thermo-electric, m

422

SUBJECT INDEX

Quantity of material, 132
Quaternary systems, 80
Quenching, 209 ff., 316

R

Recalescence, 209

Recrystallization of liardened metals,

360
Relief-polishing, 150
Residues, chemical examination of,

294 , . ,
Resistance, electrical, 252
Rontgen rays, 174
Rouge, for polishing, 140
Rusting of iron, 289
■ , protection against, 290

Salts, protecting layers of, 106

Sclerometer, 243

Scleroscope, 246

Sections, preparation of, 134

Segregation in solids, 205, 223, 325

Shakudo, 391

Silver, alloys of, 395

Sintering-point curve, 312

Skeletons, crystal, 175

Slag, in steel, 195

Slip-bands, 346

Solder, 396

Solid solutions, 7, 43 ff.

, crystallization from, 189

■ , undercooling of, 206

Solidus, 46, 305

Solubility, change of, with tempera-
ture, 54

of liquid phases, 81, 312

Sorbite, 376
Space-model, 67

, sections through, 68

Speculum metal, 391

Stage, microscope, 159

Steel, electrical resistance of, 260

, expansion of, 241

Steel, structure of, 376
Strain, structure due to, 53
Sulphides, 195
Superfusion, 197
Supersolubility curve, 200
Surface flow, 354

tension, 226

Susceptibility, magnetic, 268
Systems, number of possible, 10

Taenite, 384

Temperature, influence of, on con-
ductivity, 260
Temper-carbon, 371
Tempering, 228
Ternary alloys, crystallization of, 190

systems, representation of, 66

Thermal analysis, 8

' of ternary systems, 76

Thermit process, 109
Thermo-couples, 112

electric power, 263

of hardened metals,

359
Tie-lines, 88

Time-composition curve, 18
Tin, alloys of, 395
Transformation curves, 314
Transition temperature, 58
Triangular diagrams, 66
Troostite, 375
Twinning, 183, 345
Type-metal, 396

U
Undercooling, 197 ff. , 306

Vacuum furnace, 103
Valency, 338
Vapour-pressure, 332
Viscosity, 199
Volume, specific, 232

NAME INDEX

Figures in italics refer to the Appendix

Adams, F. D., 34.5

Adams, L. H., 232

Adler, E., 211

Andrew, J. H., 311, ild

Andrews, T., 8, 286

Archbutt, S. L., 8, 40i

Arnold, J. O., 7, 8, 126, 218, 375, 378

Arpi, R. , 29

Arrivaut, G., 4O0

Ashley, H. E., 405

Asteroth, P., 272

Aten, A. H. W., 69, 336

Aubel, E. van, 236, 263

Auerbach, F., 245

Avenarius, R., 264

B

Baikoff, A., 187, 196, 257, 374, iOS

Baker, T. J., 297

Bancroft, W. D., 9, 68, go, 310

Baraduc-MuUer, L., 106

Barlow, W., 181

Barlow, W. E., 416

Barrett, W. F., 209, 259, 274

Bartells, G. C, 291

Barus, C., 6, 229, 254, 287, 359

Bauer, O., 107, 196, 286, 325, 376,

379, 41i
Baumhauer, H., 179
Behr, G. E., 287
Behrens, H., 8
Beilby, G. T,, 53, 140, 354, 356, 357,

358, 360
Bell, J. M., 80
Belynsky, S. W., 409
Benedicks, C, 29, 148, 158, 185, 211,

213, 225, 249, 259, 285, 368, 372,

375, 379, 385
Bengough, G. D. , 226, 360, 388
Bennigson, F. , 108, 414
Bent, L. N. , 289

Berthelot, M. , 359

Berwerth, F., 384

Bijl, H. , 5S, 239, 279, 312, 40S

Bijlert, A. van, 310

Blough, E., 318

Blount, B., 157

Blyth, T. A., 2

Boecke, H. E. , 384

Bonnerot, S., 218

Bornemann, K., 11, 262

Bossuet, P., 405

Bottone, S., 248

Boudouard, O., 195

Boynton, H. C. , 245, 375

Braune, H., 195

Breuil, P., 148

Brinell, J. A., 245

Broniewski, W. , 263

Brown, J. H., 289

Brown, W. , 259, 274

Browne, A. W., 69

Brunck, O. , 404

Bruni, G. , 9, 4/g, 69, 222, 406, 417

Brunner, E., 288

Burgess, G. K. , 121, 123, 125

Buss, A., 99

Callendar, H. L., 120
Campbell, E. D., 298, 379
Campbell, W., 8, 152, i8i
Carpenter, H. C. H., 8, 59, 62, 99,

117, 225, 362, 378, 390, 392, 406,

m, 416
Cartaud, G., 140, 182, 271, 317, 349,

350, 354. 369
Ceccarelli, O., 293, 417
Cederholra, A. M., 289
Cesarjs, P. de, 414
Charpy, G., 8, 70, 218, 226, 241, 367,

368, 371, 389, 397, 417
Chaucer, G., s
Chikashig^, M., 408, 409

423

424

NAME INDEX

Chwolson, O. D., 242

Clamer, G. H., 313

Clay, J., 261 .

Coehn, A., 284

Cohen, E.. 215, 239, 285

Colson, A., 220

Colver-Glavert, E., 148

Coppet, de, 22

Cowper-Coies, S., no, 219

Cross, W., 187

Curie, M. S., 268, 271

Curie, P., 270

Curry, B. E., 293, 316, 392, 401

Cushman, A. S., 289

D

Daubr^e, G. A., 345

Day, A. L,, 112, 114, 132, 264

Deckert, 99

Degens. P. N. , 401

Dejean, P., 126, 305

De la Rue, W., 178

Desch, C. H., 178, 187, 204, 2i6,

238, 307. 320. 325. 337. 4i5
Dewar, Sir J., 261, 264, 270
Diesselhorst, H., 262, 264
Dobbelstein, 382
Dobereiner, J. W. , 22, 296
Doerinckel, F., 48,401,403, 404,411,

413
Donnan, F., 23
Dohski, L., 406, 408, 414, 415
Drude, P., 251
du Bois, H., 265, 268
Dumas, L., 272
Dumont, E., 272
Duastan, W. R., 289

E

Edelmann, M, , 213

Edwards, C. A., 8, 62, 311, 377, 390,

392, 407, 416
Efremoff, N. N., 416
EUingen, K., 383
Ericson-Aur^n, T., 288
Erman, 209
Ewing, J. A., 152, 180, 268, 274, 346,

35°. 358
Eyk, C. van, 215, 285, 310

Fahrenheit, G. D. , 197
Faraday, M., 219, 358
Fassbender, H., 272

Faust, O., 358

Fawsitt, C.'E., 199, 286, 359

Fay, H., 333, 405

FedoroSf. A. S., 406

F^ry, C, 122, 123

Field, S., Ill

Findlay, A., 6, 23, 24, 81, 239, 336

Fischer, F., 284

Fleming, J. A., 261, 264, 270, 272

Foerster, F., 378

Forbes, J. D., 344

Forsythe, W. J,, 103

Fraenkel, W., 384, 400

Frankenheim, 123

Franz, R., 262

Fr^minville, R. de, 246

Fremont, C, 349

Friedrich, K., 9, 31, 57, go, 91, 98,

99, 108, 308, 399, 400, 403, 407,

413, 414, 416
Friend, J. N., 289, 340
Fromm, O., in, 284

Gait, A., 297

Gautier, H., 12

Geer, W. C, 69

Gibbs, W.,6, 66

Gillson, G., 405

Giolitti, F., 61, 151, 218, 293, 318,

409, 417
Giraud, 204
Gladstone, J. H., 297
Goerens, P., 29, 69, 147, 173, 211,

362, 366, 370, 382, 383
Goldschmidt, H., 109
Goldschmidt, V., 179
Gontermann, W., 382, 400
Gore, G,, 209
Goulding, E., 289
Gowland, W. , 8
Gray, A., 272
Gray, J. G., 267, 272
Green, C. F., 324
Greenwood, H. C., 109
Grenet, L. , 241, 367
Griffiths, C, 21B
Griffiths, E. H., 120
Groth, P., 189, 371
Grube, G., 35, 404, 405, 411
Griineisen, E., 262
Guertler, W., 11, 225, 253, 255, 260,

270, 295, 317, 401, 410, 413, 415
Guillaume, C. E., 272
Guillemin, G., 8
Guillet, L., 191, 218, 293, 382, 383,

392, 412
Gulliver, G. H., 349

NAME INDEX

425

Guthrie, F. , 16, 22, 334
Gutowsky, N., 311, 364
Gwyer, A. G. C, 392, 400, 403, 404,
406,407

H

Haber, F,, 333

Hadfield, R. A., 7, 8, 259, 271, 272,

274, 382
Hagenbach, E , 226
Haken, W., 265
Hall, J., 345
Hallock, W., 219
Hampe, W., 195
Hannover, H. J., 152
Harker, J., 98, 99, 112
Harkort, H., 100, 119, 362, 415
Hartley, H., 200
Haupt, E., 266, 272
Heimstadt, O. , 162
Heraeus, W.'C, 98
Herschel, J., 354
Herschkowitsch, M., 277, 297
Hess, G. H., 297
Heteren, W. J. van, 4O0
Heusler, F. , 266, 272, 273
Heycock, C. T., 6, 7, 12, 21, 31, S7i

61, 69, 83, 120, 147, 174, 303, 31 T,

318, 330, 403, 406, 409^
Heyn, E., 107, 147, 161, 167, 180,

194, 196, 204, 286, 325, 34S, 368,

376. 379. 415
Hilpert, S. , 148

Hindrichs, G., 400, 40S, 403, 406, 407
HofF, J. H. van't, 43, 329
Hoffmann, F., 66
Hofman, H. O., 324
Hofmeier, F. , 91, 285, 416
Hogg, T. W., 272
Holborn, L., 112, 121, 123, 264
Holzmann, M. , 251
Romberg, 220
Honda, K., 273
Hooke, R., 2, 354
Hopkinson, ]., 260, 272
Howe, H. M., 7, 59, 185, 229, 362,

369. 376
Howe, P. E., 291
Howell, J. H., 220
Hudson, O. F., 226, 388
Hulett, G. A., 224
Humfrey, J. C. W., 103, 152, 180,

226, 3SO, 363
Humphreys, W., 334
Huntington, A. K., 145, 178, 204,

32s. 425
Hutchinson, G. A., 200
Huxley, T. H., 344

Iddings, J. P., 187

Iljin, N., 368

Isaac, E., 402. 403, 410, 415

Isaac, F., 200, 203, 206

Ischewsky, 146

J

Jackson, H., 157

Jaeger, W., 262, 264

Janecke, E,, 34, 40, 66, 73, 76, 128,

400, 410, 416, 417
Jaggar, T. A. , 244
Jefferson, J., 8
Jeriomin, K., 304, 400
Johnston, J., 232, 357
Jones, B. M,, 200
Jones, E. T., 269
Jowett, H. A. D., 289
Juptner von Jonstorff, H. von, 6

K

Kahlbaum, G. W. A. , 233

Kahlenberg, L. , iii

Kaiser, W., 387

Kamensky, G., 6, 257

Kaneko, K., 194

Kanewsky, I., 400

Kapp, A., 6, 400

Karsten, 91

Keeling, B. F. E., 59, 117, 378

Kennedy, M. B. , 379

Kerp, W., Ill

Kick, 34S

Klaassen, H. G., 268

Klein, D. , 291

Knowlton, A. A., 272

Kobayshi, M., 404, 405

Koch, 199

Koenigsberger, J., 265

Konstantinoff, N. , 189, 206, 236, 40S,

410, 415, 416
Kremann, R., 91, 285, 303, 313, 336,

416
Kriloff, B. B., 69
KroU, A., 381
Krumbhaar, W., 194
Kurbatoff, W. J., 146, 148, 375
Kurlbaum, F., 121
Kurnakoff, N. S., 9, 34, 35, 48, 52,

86, 115, 152, 189, 206, 236, 244,

249, 251, 339, 400, 401, 40g, 404,

405, 408, 410, 411, 412
Kusnetzoff, A. N.. 86, 408

426

NAME INDEX

Laar, J. J. van, 49, 331

Lambert, B., 289

Lange, E. F., 176, 223

Lantsberry, F. C. H., 8

Laschtschenko, P. N., 281, 4i.3

Laurie, A. P., 277

Lautsch, 41a

Law, E. F., 173

Lebeau, P., 40a, 409

Le Chatelier, A. , 360

Le Chatelier, H., 6, 8, 9, 118, 141,

148, 151, 161, 167, 194, 21T, 213,

238, 240, 246, 331
Ledebur, A., 372
Lehraann, O., 175, 176, 345, 348
Leon, J. T., 88
Lepkowski, W. von, 58, 399
Leroux, A., 57, 91, 108, 399, 403,

413, 414, 416
Levi-Malvano, M. , 390, 417
Levin, M. , 30, 271, 317, 400, 401
Levol, A., 5, 22, 198
Lewis, G., 333
Lewkonja, K., 64, 403, 403, 405, 409,

415
Liebenow, C, 252
Liebknecht, O., 265
Lincoln, A. T. , 291
Loebe, R., 417
Longmuir, P., 8
Lorentz, H. A., 336
Lossew, K. , 412
Lowitz, J. T., 198
Luginin, W. F. , 297
Lussana, S., 341
Luther, R,, 239, 283, 284

M

Maclean, M., 360

McWilliam, A., 218

Maey, E. , 234, 236

Mallock, A., 349

Mannesmann, R, , 218

Marantonio, M., 390, 417

Marsden, R. S., 220

Martens, A., 4, 161, 167, 244, 250

Mascarelli, L., 69

Masing, G., 222, 411

Mathewson, C. H., 86, 399,403, 404,

408, 413
Matthiessen, A., 6, 53, 58, 234, 238,

251, 254, 260
Matlonet, F. , 99
Maurer, E., 248, 275, 373, 375
Maxwell, J. C, 66
Mazzotto, D., 331, 401

Meerburg, P. A., 89

Meneghini, D., 222

Metz, de, 264

Meyer, E. , 246

Meyer, G. , 332

Meyer, H., 366

Meyer, S., 265

Mlers, H. A., i8o, 200, 203, 206

Milton, J. T., 391

Monkemeyer, K. , 404, 413

Moody, G. T., 288, 290

Morris, D. K. , 270

Mugge, G., 345

MilUer, A. , 103

Muller, P., 262

MuUer, W., 131

Muir, J., 352

Mylius, F., Ill, 284, 378

N

Nacken, R,, 63, 64

Nernst, W., 278

Neville, T. H,, 6, 7, 12, 21, 31, 57,
61, 69, 83, 120, 147, 174, 303, 311,
318, 330, 337, 403, 406, 409

Nicolson, J. H., 345

North, E., 333

Oberhoffer, P., 103
Offer, 22
Ogg, A., 414
Omodei, D. , 242
Orthey, M. , 382

Osmond, F., 2, 4, 7, 8, 126, 140, 150,
182, 218, 271, 317, 349, 350, 354,

369. 374. 375. 384
Ostermann, H., 275
Ostwald, W., 198, 200, 239, 283

PaiUot, R., 263

Palmaer, W., 288

Parkes, 91

Parravano, N., 69, 80, 401, 414, 416,

417, 418
Paschsky, N. P., 281, 413
Pattinson, 31
Pellini, G., 404, 400, 413
Perkin, F. M., no
Perrot, L., 263
Person, 209, 296
Petrenko, G. J., 31, 91, 399, 400, 402,

403, 404, 405, 406

NAME INDEX

427

Pfaundler, L., 22
Pfeiffer, V. O., 132
Philip, A., 393
Philips, M,, 294
Pick, H., 287

Plato, W., 13, 48, loi, 124, 309
PodkopaefF, N., 413
Ponsot, A., 23
Pope, W. J., 181
Portevin, A., loi, 227, 360, MXi
Pbschl, v., 244
Potdar, G. N., 90, 91
Poynting, J. H., 357
Pratt, L., no
Preuss, E. , 246

Pushin, N. A., 55, 152, 249, 281, 4O0,
401, 402. 404, 405, 411, 412, 413

Quasebart, C, 403

Quercigh, E., 9, 399, 404, 406, 412,

417
Quincke, G., 177

Ramsay, Sir W., 6, 332

Raoult, F. M., 6, 20

Rauschenplat, G. von, 262

Rayleigh, Lord, 140, 252, 354

Read, A. A., 378

Reaumur, R. A. F, de, 2, 21

Reichardt, G., 256

Reinders, W., 277, 413

Retgers, J. W., 235

Rhodin, G. J. A., 291

Richards, T. W. , 287, 333

Richarz, F. , 272

Reimsdyk, A. D. van, 198

Reitzsch, A., 263

Riley, J., 132

Rinne, F., 384

Roberts- Austen, W. C, 7, 8, 52, 115,

117, 194, 198, 216, ii8, 220, 334,

361, 391, 395, 401, 4114
Rogers, F., 349, 375
Rbhl, G., 195
Romanoff, L., 82, 403
Roozeboom, H. B., 7, 12, 13, 43, 44,

53. SS. 59. 69, 363
Rose, T. K., 52, 401
Rosenhain, W., 8, 70, loi, 125, 142,

14s, 152, 160, 163, 180, 212, 302,

307, 346, 348, 363, 401
Ross, A. D., 267, 272
Roth, K., 232
Rowland, W. S., 293

Royston, G. P., 379

Rudberg, F., 401

Rudolfi, E., 295, 409

Rudorff, F. , 21, 22

Ruer, R., 53, 72, 194, 271, 336, 368,

383, 401, 413, 415
Ruff, O., 369
Ruskin, J., 344
Rykowkoff, A., 50, 53, 402

Sabersky, E. , 211

Sackur, O., 287

Sahmen, R., 52, 76, 131, 241, 402,

408, 409
Saklatwalla, B. , 362
Saladin, 118
Sand, H. J. S., 284
Sander, W., 413
Sandonnini, G. , 9, 417
Saniter, E. H. , 210, 378
Saposchnikoff, A. V., 400
Sauveur, A., 28, 324
Schaffgotsch, F. S. , 123
Schemtschuschny, S. F., 48, go, 52,

53, 208, 244, 249, 402, 406, 409,

413, 415, 416
Schenck, R., 251, 253
Schepeleff, ]., 415
Schleicher, A. P., 400, 417
Schoen, P., 414
SchoUer, A., 333
Schbne, G. , 378
Schreibers, 3

Schreinemakers, F. A. H,, 69
Schroder, I. , 33r
SchiikarefF, A., 297
Schiiller, A. , 34, 410
Schukowsky, G. U.,409, 411
Schulze, F. A., 263
Schiiz, E. , 415
Senkowsky, N. , 249
Shepard, C. U., 371
Shepherd, E. S., 9, 62, 69, 70, 318,

388, 400, 404, 410
Shore, A. F. , 246
Siedler, P., 232
Sieverts, A., 194
Sirovich, G., 69, 80, 418
Smirnoff, W. J. , 251
Smith, D. P., 403, 408, 411
Smith, G. McP. , 333, 334
Smith, S. W. J., 284
Smits, A., 369
Soiby, H. C., 3, 185
Sosman, R. B., 114, 132
Spencer, L. J., 371

428

NAME INDEX

Spring, W. , 30, 82, 209, 217, 219,

232, 286, 331, 343, 403
Springer, J. F., 246
Stansfield, A., 115
Stanton, T. E., 353
Stark, W., 266
Stead, J. K, 8, 131, 138, 148, 157,

160, 167, 181, 185, 187, 205, 223,

226, 379, 413
Stein, S., 4

Stepanoff, N. J., 252, JOJ, 405
Stock, A., 312
Stodart, 219
Stoffel, A., 400, 411
Stokes, G. G., 66, 88, 89
Stoney, J., 157
Stortenbecker, W. , 336
Stromeyer, C. E., 194, 379
Strouhal, V., 6, 229, 254
Sturm, E. , 232
Sucheni, A., 281, 404
Suschtschinsky, P. von, 189
Svedelius, G. E. , 240

Tafel, V. E., 62, 305, 388, 410, 412,
417

Take, E. , 272

Tamaru, S., 401, 403, 415

Tammann, G., 6, 8, 13, 18, 33, 64,
76, 86, 200, 226, 241, 270, 271, 307,
310, 317, 329, 339, 343, 358, 384,
401, 402, 403, 404, 411, 412, 415

Tavanti, G., 61, 318, 409

Teal), J. H. , 187

Tegetmeier, F. , 220

Tholander, H. , 194

Thomas, N. G., 200

Thompson, C. , 66, 88, 91

Thomson, J. C, 289

Topler, M., 242

Tomh'nson, H., 272

Traube, I., 249

Treitschke, W., 206, 40S, 41b

Tresca, 343

Tschernoff, D. , 5, 176

Tucker, P. A. , 70, 142, 302, 401

Turner, T. , 243, 248, 359

Tyndall, J., 344

U

Upton, G. B., 378

Urazoff, G. G., 50, 53, 250, 402, 407,
409

Valentiner, S., 121

Vanstone, E., 410

Vegesack, A. von, 76, 403, 415, 417

Veil, T., 273

Vincentini, G., 242

Violle, J., 220

Viviani, E., 401, 417

Vogel, R., 39, i8s, 310, 400, 402,

404, 405, 407, 417
Vogt, C, 251
Voss, G., 40, 402, 403, 407, 408, 411.

412, 415

w

Wahlberg, A., 246

Wahl, W., 4O0

Waidner, C. W., 123

Walker, W. H., 289

Wanner, 121

Warburg, E., 220, 274

Warington, R., 209

Wark, N. J., 365, 374

Watts, O. P., 403

Wedding, H., 4, 131

Wedekind, E, , 273

Weinschenk, E. , 371

Werth, J.,4

Weyl, F., 218

White, W. P., us

Whiteley, W., 406

Whitney, W, R. , 289

Widmanstatten, A. J. F. X. von, 3,

383
Wilm, A., 396
Wiedemann, G, , 262
Wien, W. , 121
Williams, R. S., 401, 403, 405, 409,

411, 413
Willows, R. S. , 253
Wills, A. P., 265
Wlrkner, C. G. von, 21
Wohler, F., 350
Wogau, M. von, 334
Wologdine, S. , 52, 106, 115, 196
Woods, S. H,, 392
Wright, C. R. A. , 68, 82, 88, 91
Wust, F., 9, 103, 126, 173, 382

Yerxa, R. B., 324

INDEX OF SYSTEMS

Figures in italics refer to the Appendix

Aluminium-antimony, 33, 236, 40i

-bismuth, 405

-cadmium, 403

-calcium, 406

-chromium, 40G

-cobalt, 4m

-copper, 274, 294, 392, 401

-copper-manganese, 272

-copper-tin, 416

-copper-zinc, 41S

-gold, 144, 406

-iron, 404

— ^ -lead, 81, 403

-magnesium, 265, 395, 404

-manganese, 401

-nickel, 407

-potassium, 403

-silicon, 4O0

-silver, 40&

-sodium, 405

-thallium, ^3

-tin, 4O0

-zinc, 404

Antimony-arsenic, 41i

-bismuth, 282, 401

-bismuth-copper, 417

-cadmium, 206, 40S

-cadmium-copper, 4iV

-calcium, 415

■ -chromium, 409

-cobalt, 409

-copper, 144, 184, 187, 257,

409

-gold, 39, 309, 404

— — -iron, 236, 410

-lead, 234, 39S, 4O0

-lead-tin, 131, 411

-magnesium, 405

-manganese, 411

■ -nickel, 283, 412

-palladium, 413

-silicon, 401

-silver, 404

-sodium, 412

-tellurium, 265, 405

Antimony-thallium, 40!)

-tin, 395, 413

-zinc, 208, 236, 413

Arsenic-bismuth, 405

-cobalt, 414

-copper, 401

-iron, 414

lead, 414

-manganese, 414

-nickel, 414

-platinum, 414

-silver, 414

tin, 414

■ -zinc, 414

Bismuth-cadmium, 234, 4O0

-cadmium-lead, 41H

-cadmium-lead-tin, 41S

-cadmium-tin, 411

-calcium, 414

-chromium, 403

-cobalt, 405

-copper, 351, 409

-sold, 4O0

-iron, 403

-lead, 186, 296, 4O0

-lead-mercury-tin, 296

-lead-tin, 70, 191, 427

-magnesium, 404

-manganese, 273

-mercury, 400

-nickel, 408

-potassium, 40S

-silicon, 405

-silver, 234, 599

-sodium, ^8

-tellurium, 265, 404

-thallium, 222, 408

-tin, 186, 234, 4O0

-zinc, 82, 405

Cadmium-calcium, 408
-chromium, 403

-copper, 408

■ -gold, 401

429

430

INDEX OF SYSTEMS

Cadmium-iron, 403

-lead, 73, 234, 4O0

-lead-mercury, 73, 411

-lead-tin, 417

-magnesium, 404

-magnesium-zinc, 411

-mercury, 55, 280, 4i>:i

-mercury-sodium, 417

-nickel, 415

-potassium, 408

-silver, 406

-sodium, 86, 408

-tellurium, 404

-thallium, 400

-tin, 225, 234, 400

-zinc, 400

Caesium-mercury, 409
Calcium-copper, 415

-iron, 403

lead, 415

-silicon, 415

-thallium, 415

-tin, 415

-zinc, 408

Carbon-iron, 7, 59, 209, 217, 223, 228,
241, 259, 361 ff.

-iron-manganese, 382

-iron-phosphorus, 383

-iron-silicon, 382

Chromium-cobalt, 402

-copper, 403

-iron, 415

■ -lead, 403

-nickel, 402

-silver, 402

-tin, 403

■ -zinc, 403

Cobalt-copper, 256, 402

gold, 400

iron, 401

lead, 403

nickel, 270, 401

phosphorus, 415

silicon, 409

silver, 403

thallium, 403

tin, 409

zinc, 415

Copper-gold, 52, 249, 395, 402

gold-silver, 416

iron, 131, 394, 402

iron-manganese, 417

iron-manganese-nickel, 418

iron-nickel, 417

lead, 83, 397, 403

lead-silver, 91, 416

■ lead-tin, 417

magnesium, 409

manganese, 50, 394, 40.2

■ manganese-nickel, 417

Copper-nickel, 48, 222, 249, 255, 262,

393. ^1

nickel-zinc, 305, 394, 417

— oxygen, 204

phosphorus, 185, 204, 325,

397. 415

phosphorus-tin, 393

palladium, 401

-platinum, 401

silicon, 294, 409

■ silver, S7i 183, 186, 249, 256,

39S. 399

tellurium, 409

thallium, 403

tin, 61, 236, 250, 293, 318,

390, 409
tin-zinc, 191, 389

-zinc, 62, igo, 221, 225, 227,

287, 290, 297, 387, 410

Gold-iron, 402

lead, 216, 407

magnesium, 407

nickel, 271, 400

palladium, 401

palladium-platinum, 73

platinum, 48, 401

——-silver, 217, 235, 249, 254, 395,
401

sodium, 404

tellurium, 404

thallium, 30, 400

tin, 407

zinc, 407

Indium-lead, 401

thallium, 402

[lidium-platinum, 235
Iron-lead, 403

manganese, 271, 380, 401

molybdenum, 415

— -nickel, 225, 271, 383, 415
phosphorus, 205, 224, 415

— -platinum, 219, 415
silicon, 410

silver, 402

— -thallium, 403

tin, 410

tungsten, 415

— -vanadium, 402
zinc, 219, 415

Lead-magnesium, 404

magnesium-tin, 76, 417

-manganese, 403

mercury, 234, 400

nickel, 403

palladium, 413

platinum, 283, 413

potassium, 411

INDEX OF SYSTEMS

431

Lead-silicon, 4SiS

■ -silver, 31, 234, 4O0

silver-tin, 416

silver-zinc, 91, 416

■ sodium, 397, 412

tellurium, 405

thallium, 40i

tin, 29, 234, 262, 284, 396, 401

tin-zinc, 87, 411

— - -zinc, 82, 40S
Lithium-tin, 411

Magnesium-nickel, 40, 411
■ — — -potassium, 40S

-silicon, ^3

-silver, 265, 40Q

-sodium, 403

-thallium, 411

-tin, 35, 40i

-zinc, 405

Manganese-nickel, 53, 40S

-phosphorus, 416

-silicon, 411

-silver, 251, 402

-tin, 411

Mercury-potassium, 34, 42, ilO

-potassium-sodium, 411

-rubidium, 411

-silver, 414

-sodium, 34, 410

-tellurium, 404

-thallium, 281, 404

Mercury-tin, 234, 4O0

-zinc, 400

Nickel-phosphorus, 186, 416

-silicon, 413

silver, 403

thallium, 403

tin, 412

zinc, 412

Palladium -silver, 401
Platinum-silver, 404

-tin, 41S

Potassium-sodium, 404

-thallium, 411

-tin, 411

-zinc, 411

Selenium-silver, 413
Silicon-silver, ^X)

-thallium, 403

tin, 401

Silver-sodium, 399
Silver-tellurium, 406

thallium, 4O0

tin, 236, 404

— zinc, 236, 406

Sodium-tellurium, 412

■ -thallium, 412

tin, 412

-zinc, 412

Tellurium-tin, 40.5

-zinc, 40b

Thallium-tin, 401

-zinc, 403

Tin-zinc, 234, 401

THE END

PRINTED BY

WILLIAM CLOWES AND SONS, LTMITF.O,

LONDON AND BECCLES.